Question

1988 Buick Regal
I'm having engine hesitation and very rough performance. The exhaust sounds like it's saying "DougDougDougDougDoug". My brother-in-law recently replaced the fuel injectors on the advice of a well certified mechanic. The car ran much better for a few weeks and now this problem has quickly creeped up again. It is idling down to almost a stall and then back up slowly. The MAF Sensor has been replaced and appears to be in good condition. When fuel injectors were replaced, a high-pitched "vaccuum" sound was heard, but unidentifable. When accelerating, the vehicle shakes at low pedal pressure, but smoothes out / vibrates much faster when the vehicle downshifts. Just recently the vehcle has had markedly reduced acceleration...pressing the pedal farther does not correspond to more acceleration.
Thoughts?
(Thanks!)