i would check the cam and lifters as well, if a lifter has decomp'd or the profile has worn considerably I would guess that once the engine is up to temp and the oil visc has thinned they would not function correctly and the valves would not open hence the 3 cylinder synario. Whip out the cam and lifters and take a pic.

I have a sneaky feeeling this may be the problem. Never seen a zvh with the oil galleries unblocked??? though it would leak everywhere?? Very strange, makes you think what else has not been done to normal practice ???