This is a good point. If the vacuum canister on the LOM distributor has gone bad, it will not advance properly...or at all. Also the springs in the distributor on that end is worth a look.I am not that knowledgeable on the vacuum only advance distributors, but will a vacuum leak or malfunction cause this type of issue? It would at the least cause the timing to be retarded.
Vacuum canister,if applied suction(brake bleeder pump use )the vacuum canister linkage moves but doesn't hold any pressure,this is faulty,correct!!
I didn't see any thing about changing out the coil? This is the type of problem a bad coil will give you. Starts and runs fine until it gets hot.Changing cap and rotor and putting some new wires in distributor..