156 2.0 TS, engine not running smooth
I'm having a problem with my 156 2.0 TS model '99. After the 80k maintenance the engine started working uneven. Idle RPM went up and down and rarely it even shut-downed. On the 90k I've taken it to an official AR garage which checked the computer and found no error codes. Since it was the time for timing belt I've changed this as well. After that the car went worse... The idle RPM are even more unstable, the engine is shut downed more often, trying to drive on 20-40 kph (e.g. traffic jam) became like riding a wild horse... the car is jumping like a young horse and the worst part is that there is a power jump on 4200 rpm (more or less). The car is like choked underneath this rpm and then suddenly it open's and produce the power you would expect.
I would appreciate any advice to identify and solve this issue.
I'm pretty much OK with mechanics (however my previous passion was GT Junior '72...) so please be as detailed as possible.
Thanks in advance.