[s-cars] New timing belt is in - now car is acting up

[radek at uniserve.com]

Hello again;

This is a 1996 UsS6; timing marks align ok.  The only things that were  
changed are the belt with rollers, water pump and valve cover gasket.   
Also, the intake manifold was removed and put back.

The car ran fine for about 10-15 minutes, then spat out the half-moon  
from under the valve cover gasket and started acting up.  We put the  
half-moon back and it's staying in place, but now the car is bucking  
under 3000 rmp.  Over that it runs smooth.  Idle is now significantly  
lower than before (it used to be too high) and uneven.  No codes  
whatsoever.

I'm thinking crankcase pressure (pushed the half-moon out), perhaps  
misfires (but no codes). Where to look first?  Can you set base timing  
on this car, since it has no distributor?
TIA

Radek.