drifting frequency TOE7706

TOE7706 is a 12MHz frequency generator from Toellner. A clean signal is 
coming out, amplitude is ok, but unfortunately the display does not 
always show a stable frequency.

when I wanted 547kHz the display suddenly started to move to 548, 549, 
552, 547, 550, 552, 547 and so on. Its not always stable.

Transformer seems to get quite warm. No bulging caps to see. Filter caps 
seem ok. No damage to see easily. The voltages on the main board left to 
right: 5.0, -5.19, 11.39, 4.92, 11.94, -11.93, -24.1, 24.0, 23.9, 0, 
1.6, 0, -23.9.

did anyone have such a problem yet and may have an idea how to proceed 
here?

it would be sad not being able to repair.