Any audi 4000 people - got an issue

Figured since this board has been good for almost every kind of question - may as well try a car issue

Got a audi 4000 quatro (2.2 I5) - here is the issue:

Starts up perfecty and revs nicely.. can start driving it with no issues at first. Once the needle for temp starts to go up it starts missing and running very rough. If I rev out the gear at around 5K it clears up till the revs drop. Once it gets up to running temp - it will clear out and run great for miles and miles.

Did the basics - new plugs/wire/cap/rotors. Changed the temp switch that sends to the ECU. Since it runs so well warmed up i assume the O2 sensor is doing well. I am stuck - anyone?

they are fun cars when they run correctly!

This brings back memories! It's an old Audi, so my money is on vacuum leak or corroded electrical connectors/grounds. I've forgotten everything I knew about CIS-E, but that's all that ever goes wrong, right? :)

Disconnect the O2 sensor to force it into open loop mode and see if that changes something. That'd at least give you something more to think about.

Or the thermo-time switch, maybe? Cold start injector leaking?