'94 Ranger thermostat gone wild

Hi,

Old problem that I can't resolve. I tried in here a couple of years ago, but all suggestions--to this point--have done no good.

The story:

With a regular thermostat installed (195 deg), the engine will get up to warm in about 5 to 10 minutes. What then happens is that the needle on my gauge begins to do a windshield wiper manuever. It will go up past the "L" in NORMAL, and then sudenly (when the thermostat opens) drop down to almost the "C" on the gauge. Now, the problem with that temperature thermo is that when it's at its zenith, the engine's idle will raise. It's really noticeable if I'm at a stop, but it will go that far even when I'm driving on the freeway. It's a constant cycle that lasts about a minute or so.

If I put in a 180 deg thermostat, it will do the same thing, but the high point will only reach about the middle of the gauge...no raising of the idle. It will still go down to fully cold on the left swipe of the cycle. My friend's scope still says the temp is near 200 deg at this point, but the idle is solid.

We have tried a new (two, actually) idle sensor, we have changed out the entire emissions system (for other reasons), and have pulled a few hairs out.

My friend--the mechanic, I'm a schmo--now thinks that mabe the "fins" of the water pump may be going bad. But this has been going on for the better part of two years, so I would have thought the entire pump would have failed in that time.

Can anyone PLEASE offer any suggestions? I'm really just trying to get this beast running well enough to sell!

The specs on the truck: '94 Ranger

tia, jm