Air leaks that fix themselves on first cycle always make me think valve, but could also be the cylinder.
It can be hard to differentiate, leaky cylinders can blow back through the valve, making it look like the leak is there.
An intermittent problem makes finding it that much more fun. If you can bypass the valve and put air straight to the
cylinder you might be able to isolate the problem. If'n it was me though and the valve wasn't particularly costly I'd
probably just swap a new one in, if it fixes the problem great, if not you have a known good spare, and start looking at the cylinder.