OBD1 car so we can pretty much forget getting help from an onboard system (I have not been helped very much by the ODB1 system). We can look at what happens when a car warms up, what comes to mind is it leans out. So saying this problem is caused by a small vacuum leak and the system its self compensates while car is cold would be fine if we only have issues at idle, but we have them at speed.
I am still thinking of a mixture problem that turns into a flat out miss when warm. Not to put the cart in front of the horse my thoughts are based on the idea that no primairy or secondary ignition issues exists (in laymans terms "the car does not need a tune-up"). That being said perhaps an injector with a really bad spray pattern is the cause. This may be one of those rare times an injector cleanse would work out as being worth the money. I repeate, you must make sure all tune-up items are taken care of before getting into the fuel system.