Old Pro Reverb

[Tommy]

This pro reverb has been gigged - it works ok great tone and volume but then other times it dosnt have the power or punch.
Ive changed the caps.

Where can I look next?

[stereomonostereo]

I assume you've tested your tubes? Could be a drifting plate resistor as well.

[pdf64]

As above, but check all resistors for drift (cathode etc can drift too).
See
Tube Amplifier Debugging Page
Re-flow all solder joints, including those difficult ground ones.