94 Beretta has rough idle and can not maintain constant speed above 50 w/o shaking. Service engine soon light not on and OBDI scan resulted in 0 results leading to conclusion that O2 sensor okay. Convertor not rattling and changed fuel filter. Any suggestions on what may be wrong?

Almost forgot...checked plugs...theyre okay. This is the 3rd time in 2 years this problem has occured. Other two times, went away instantly after period of 7-10 days

my mom has a 94 beretta Z26 and it does the same exact thing.. she doesnt no whats wrong with it either.. if i new i wish i could help ya!

Sounds like a weak or dead cylinder caused by a bad/weak plug, plug wire, coil pack, etc. A 4 cyl will shake like the devil with a fouled plug and a 6 cylinder will not do much better, especially at higher speeds. Had this with my son's Corsica, was a problem with a plug wire that would short on occasion, and the other son's Explorer did this twice with plugs that became fouled. A tune up would be in order to rule out coil pack(s).