[cwc]

After some new caps, a new tube and an alignment my 38-12 seemed to be working fine so I put it in the living room  to see if it would run all day without a problem.

Well, it didn't.  After 2-3 hours the local oscillator changed frequency.  It didn't "drift", it was an abrupt shift down in frequency by about 35 Kc.  I repeated the test a few times and it repeated the shift consistently.

I assume this is a heat problem and know how to test the various components to track it down, but I thought I'd ask if this is a known problem with a known solution to save a bunch of tedious testing. 

Any suggestions will be appreciated.