I seriously doubt it is the plugs or the injectors.

Lenny is probably on the right track. I'm going to guess that you replaced the EHA (cold start valve) and you now need a mixture adjustment.

You don't need an exhaust analyzer. http://www.landiss.com/mixture.htm

That site explains the procedure.

Try setting mixture around 45%.

When exactly does the car stumble? Time from start, engine temp etc?

__________________
'90 300SE 298k
-300K and it gets put into retirement.
'80 300D 255k Purchased new by family in 1980.

Had a:
1973 220 (gas)
1980 300SD
1992 400E