The Reverb tube (small tube nearest the large transfomer and near the RCA jack for Reverb Send) should be a 12AT7 type. a 12AX7 used here might exhibit intermittent behavior. Also, it COULD be a faulty 12AT7. Check that and let me know. If it were something like a bad Reverb transformer, that is covered under warranty and would mean no charge (other than shipping) for the complete upgrade.
Bruce's reply gave some more information on which tube sent the signal to the reverb, but nothing about the time required to do the upgrade once he'd got the amp.
I went back to the amp, took the back grille off (remember, all this time I'm still using the amp for weekly band rehearsals) and tried to match up his description with the positioning of the valves. See part 11 for my findings.