Get some tubes, it's almost surely your problem. I'd discontinue use until you get a new set in.
Originally Posted by ShawnF
That's really not true. 30-60 seconds of filament heat should be more than enough to ensure correct operation. You just don't want to hit a cold cathode plate with high voltage.
As for whether the tone drifts slightly after the circuits have all warmed up in 5-10 minutes... I can believe that as most of the electrical components will drift in their specifications with temperature (slightly), e.g. capacitors.
You also do not need to switch to standby before powering off.