Hmm... will have to consult with Mike on that.

RE: vacuum... ssalonga: have already been along that route, and plugged some vacuum leaks, but I want to have my fittings double-checked again to eliminate this... ignition system is fine... but its possible, since ignition coils are a known wear item on this engine.

I'm focused on the vaporizer since the problem seems to be related to the time of day and traffic conditions... cold running will not cause ignition coil issues or vacuum problems, but it will lower water temp, which may affect vaporizer function.

*Mile2: your case might be idle vacuum related... try to do some throttle-body cleaning and check your idle vacuum... or maybe add some timing advance (if your car has a distributor cap) to bump your power at idle a bit for LPG.