Well, it's my turn to ask for suggestions..

it seems to me that you have checked all the obvious things out. I would suspect the torque converter is mal functioning. or miss application. i have an toggle switched electric lock up in my 904 and if i leave the torque converter locked up at idle ,its noticeably lame until i remember to switch it back. no way that 170# in all cylinders could be cam, compression, timing etc. Has to be external. Collapsed exhaust?