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