Check the function of the thermal vacuum valve that is located on the front/top of the intake manifold. That valve controls the routing of vacuum to the distributor based upon engine temperature. The valve serves to function as a way to have the ignition retarded while the engine is cold, thus avoiding over-revving while the idle air valve is open. This change in ignition timing will affect performance if it does not go back to normal when the engine is warm. The proper schematic for this is kind of confusing, as it allows vacuum to bleed back into the manifold when warm, which negates the effect, while closing the bleed back when cold which then allows the vacuum to reach the distributor via a 'T' type connection. (this is if memory serves me correctly - take your chances )

Also, check the ignition timing for full advance, and watch the timing as you slowly rev the engine up to 3000 rpms to watch that the timing advances smoothly, rather than suddenly jumping i.e., if the advance weights suddenly become 'unstuck' rather than moving freely.

Please post a follow up to let me know if this resolves the problem - I'm curious...

[This message has been edited by 450 SLC 5.0 (edited 10-06-2000).]