Very weird Vibration in my 1997 mitsubishi galant v6 24v Auto

Hi all i hope someone can help
at about 50mph, the car starts to shake really badly it carries this on no matter what speed i accelerate to, but as i cruise at that speed the
car setlles down.
I have been told that this is a gearbox problem? which is confusing considering that the car is fine driving around town or anything about to about 50, which is when it starts to shake.
I took it to a mechanic who basically charged me 100GBP to tell me he cannot see what is wrong but there is no were and tear on the aluminumcasing on the 4th gear ( whatever that means)
I am hesitant to go ahead and get the gerbox rebuild due to
1) astronomical cost 2) they do not even know the gearbox is a problem.
It is the engine which is shaking, of this i am sure, but why seems to be the issue, does anyone have any advise or has anyone has a similar problem?
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OK, the symptoms are the car shakes on acceleration, and the as the engine load is reduced, the shaking stops. So the question is what's shaking? The engine or the transmission or suspension?
I'd start with the engine. Shaking under load is commonly plug leads or/and plugs. Perhaps the V6 is a bear to change these, which is why it was not done.
Less common is low fuel pressure or/and gummed or partially blocked injectors. The engine computer (ECU) may or may not report error code(s): this is the FIRST place to look for clues.
The air cleaner could need replacing. The emissions system may need evaluating, but generally ECU code(s) will point here if there's a problem.
It's unlikely to be the transmission, as there's no firm indication there's any problem there. Your's has (AFAIK) a computer as well, and this may be reporting an error code. If the trans was slipping and grabbing under load, you'd usually hear the engine revs vary up and down.
Wheels and suspension? Might be worn ball joints or tie rod ends, but the mechanic would surely have picked up those. This will cause shaking of the steering wheel but not the whole car, which happens if the engine is the cause.
If you've never had the car tuned up, get it done. Find a place that can read the ECU error codes.
If you never had the trans fluid replaced, get it done. It's time and can't hurt.
Don't go back to the same mechanic: you've been taken for an expensive ride at some level, though the guy probably spent a couple of hours or so poking around and finding nothing. At the very least for 100GBP you could have had the plug leads replaced.
Stewart DIBBS www.pixcl.com/lancerproject
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