Hi all. Just finished a 5E7 Bandmaster that sounds great until I turn up the volume and hit a hard chord, then a "chirpy" garbled sound that diminishes as the chord does. Sounds like a cold solder joint, but can't replicate it by banging on the amp. Swapped out tubes with no change. 345 VDC on the plates, biased at 40 mA. Ideas? Thanks.