From Heinzz: "Your problem could be with the Exhaust system."

That rang a bell with me. On an '87 240 with original cat (about 11 years old), the innards came loose and rolled up into a ball about 2"-21/2" in diameter. This "ball" would intermittently roll to the back (like when accelerating—but not every time) and plug the cat outlet, causing weak/rough running..

Then, when stopped, the "Cat ball" would roll free again and all was well till the next time. Sound familiar?
--
Bruce Young,
'93 940-NA (current)
'80 GLE V8 (Now gone)
'83 Turbo 245
'73 142 (98K)
'71 144 (track modified--and still here)
New 144 from '67 to '78
Used '62 122 from '63 to '67