[Test-Equipment] W&G SPM-15

[Gollum]

Anybody thoroughly familiar with the Wandel & Goltermann SPM-15 ?
Mine works ok, were it not for the following quirk.
When switched on from a cold state, initially there are no problems. After about 3 or 4 minutes, while the digital 
display still indicates the frequency where the instrument is tuned at, the internal tracking generator and the 
selective voltmeter shift to the nearest multiple of 1 kHz....
I have the service manual, but it is rather complicated, so before taking the sabbatical week it will need to be read 
and understood, I am asking here first, in the hope somebody can be of fast help....

It looks like it is a thermal-related problem, but my diagnosis stops here...

Thanks

Alberto