Weird tube power supply issues

Hi everyone, I've got a pmillett HA-2 clone. Everything has been fine, but just recently after a bit of use the 6.3v appears to gives out. However the 5v for the rectifier tube is still underway. Let it sit for a bit and everything comes back just fine. And then it goes out after a while.

I'd think if a winding were failed it would be just completely gone, but things seem to be intermittent which is odd. Any thoughts?