Dont know if you guys will remember but I always complained about my 3.8 as a flat spot on four wheels.
During the front suspension change I had to remove the strut brace (obviously). The strut brace was pushing down on the plenum quite hard and as a result has worn away some metal.
Anyway, i decided not to refit it.
All my flat spos have gone! The car drives like it should now!
What must have been happening - I have two thoughts on this one of which might be totally stupid but its a thought:
1) Plenum was being pushed down so hard that something somewhere was flexing and causing an Air leak
2) Engine was not being allowed to move and this somehow had an effect on power delivery - unlikley but its an idea.
Anyway - the lesson learnt here is this, if your going to fit a strut brace FORGET the Weichers one because it will damage your plenum.