Check engine light in 1988 Audi 90Q

[Radek]

Hello!
The check engine light comes up a few minutes after the engine is started.
It seems to be flickering a bit.  The engine seems a little rough, there is
some vibration at idle speed and the speed seems to vary a bit.  What should
I check first?  I just got the car recently, replaced spark plugs, wires,
cap and rotor, as well as installing a new air filter.  Any suggestions will
be highly appreciated.  Thanks.
Radek
1988 Audi 90 Quattro