94 Trooper Problem - desperate

My dad's got a 1994 Isuzu Trooper with right at 100k miles on it. They are getting ready to head out of town (my wife & I are as well) to the beach in
about a week, towing a travel trailer. Hence the urgency of the request if anyone has any suggestions. Here's the story...
About a month ago, they had gone on a weekend trip (towing the trailer), and on the way back, started having problems with the Trooper not acting right. He said when he began to press in on the accelerator after having let off the gas, it would tend to jump and buck and not want to run right. It did it several times, but he was able to get home without any major problems. He was thinking perhaps it had something to do with running gas that has ethanol in it (I forget the %, but such as is common for normal vehicles to be able to run).
After getting home and running it solo trying to reproduce the problem, he found that when he would stomp the accelerator, it would downshift to 2nd gear and then have very little power. He was unable to get it to reproduce the "bucking" problem.
He took it to their normal mechanic, and he immediately reproduced the problem. Apparently their computer reader won't read a vehicle as old as his, so he shorted something out to get it to give a code readout, which confirmed his suspicion that it needed a Throttle Position Sensor. After a week of fighting to get the part in and installed, it continued to have the problem. It has also had an occasional and intermittent "check engine" light. The machanic managed to catch it in the act, and diagnosed it as needing a MAP sensor. After its installation, the problem continues. The mechanic tells him that it is now acting like it is trying to flood itself. Not sure if the other symptoms have changed...
Sounds like they are about at their wits end as to what the problem could be. The Isuzu dealership would be the next logical choice, but they screwed him over once already. (Bought the vehicle there, they put on a new head gasket before it was sold...broke off some bolts holding on the exhaust stuff to the engine and basically fixed it enough to get it off the lot...then told him he had bought it "as is" and they wouldn't do anything about it). So...the Isuzu place isn't a welcome option.
Any thoughts on what might be going on? The backup tow vehicle is a 93 Trooper, with over150k...so obviously he would rather take the one that is now having problems... :-)
Thanks in advance,
Wesley
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always told me "Fix the obvious and let the rest of it take care care of itself"..... You must have (tell me you did so) check all 6 plugs. W/ 100K on the clock, if your dad is like mine, they probably have a LOT of miles on them. I'd change them. That goes for plug wires, also, and it wouldn't hurt to look at the distributer cap, if a 94 has one - I think it does. Look for a track or evidence of shorting or moisture. If you find any at all, replace it and the rotor. Again, if the 94's don't have a distributor (my 98 doesn't), then ignore this.
Your next thing to check is the timing belt - with 100K on it, and probably never having changed the timing belt, what you're describing could be a timing belt that has jumped one tooth on the cam. The 94's have a Single OHC, if I recall correctly, as opposed to my oilburning, gas-guzzling 3.5 piece of garbage engine in my 98, but I think if your tensioner wasn't adjusted right or perhaps has reached the end of the travel then it could do this. I don't know, but a quick check of the lineup of the gears should tell you if it's all in time properly.
Since your parts-changing mechanic has already done the work for you on the TPS and the MAP sensor, I wouldn't waste time there. The last thing I'd check (no kidding) is the alternator - make sure it is putting out 14.5 volts, and you have a GOOD battery that you KNOW is good. DAMHIK. Been there, done that. Car literally wouldn't run with a battery w/ one dead cell, even after being jumped (on the hottest 4th of July I've ever been in with a pregnant wife).
Last of all, I'd check for water in the gas. Try draining a little of the tank and see what you get.
And that, my friend, should keep you busy for a while. Let us know what you find on these and if you strike paydirt on any of them. Best of luck. r/ Steve

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Steve,
Thanks for the tips. Unfortunately, I'm not much of a mechanic...but I did pass your email on to dad to pass the info to the mechanic working on it.
Thanks again!
Wesley

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And the solution is........
O2 sensor. Well...that plus the TPS sensor. After the mechanic pulled the O2 sensor out, it ran like a top. So, they put a new one in it, and it still ran great. They pulled the MAP sensor out returned it, and tried to do the same with the TPS, but apparently that was faulty too. So...TPS + o2 sensor and it's fixed. Sounds like they also replaced the spark plugs and fuel filter, but at 100k, I'm sure they were both needed anyway.
Thanks,
Wesley

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