Had a strange occurance on my 383 where exhaust was suddenly blowing out of the valve cover breather hole. Took it to a mechanic and he told me the intake manifold gasket failed and allowed exhaust to be pushed into lifter galley from the exhaust crossover from heads to intake manifold. Is this a common problem?

Not common at all. I would make sure it is exhaust, and not excess blowby from hole in piston etc. Compression check may be in order.

Compression was fine. Around 140 for all eight. That was when I took it to a mechanic. He seems to have fixed it.

Excellant.