I had my motor rebuilt about 2 yrs/ 24k miles ago. Now its seems to be having problems with the cold start. It will miss and jump up and down around 800-1200 rpms. Its sounds really bad, but it gets better as the motor warms up and always goes away after the first few minutes. It doesnt do this if the motor has been ran recently and is still warm.

Any ideas?

 

What motor, year??

 

It sounds like a mixture issue to me. Your cold start injector might be malfunctioning. Your O2 sensor could be sending bad info, throwing the air/fuel ratio off.

I had a pre-warmup stutter that was solved with a new O2 sensor.

 

How old are the spark plugs? what does the inside of the dist-cap look like?

When the motor is cold, the gas is not preheated ( so to speak ) as it enters the cylinder, so it takes a good strong spark to make it burn. As the motor heats up, the gas gets heated by the motor, so it takes less spark to burn it.

Bad plug wires can also cause bad spark.