I would try a compression test and leak-down test. See if compression is holding up, or maybe the culprit is just one slug out of the bunch.
In any event, I would plan on a ring job and while you're doin the bottom end, all the crank & rod bearings and motor seals.
'84 190E 2.3
'83 Porsche 944 (track car)
'84 Porsche 944 (parts car)
85.5 Porsche 944 (sled)