I have a 94 eg coupe with all stock z6. I bought this car from the original owner two years ago. Last winter when the air was cold, you start the car up and take of and it'd have a slight studder around 1-2k. Note: this would only happen with both the outside air and engine temp were cold. Once it warmed up, no more studder. In the spring the studder stopped once the air warmed back up. Now it's cold out again, and the studder is back, but worse. Now it doesn't matter if the engine is warm or cold. Very hard to accelerate at all. Once you get going and hold a steady rpm it fine. Also, there is no studder if I'm not accelerating. Rev's smooth if I"m not moving. I think it's something timing related but have no idea. .I can set my timing and my tach will work. But then within a few days my tach stops working and I know the timing needs reset. Is the belt slipping? Any ideas?

 

Few things to start you out right:

You listed car. Good
You stated your engine. Good
Your mileage is somewhat important
Any parts you have changed since you got the vehicle