Thanks @coromonadalix. After measuring all DC voltage rails with nothing amiss I set about re-seating all plug-ins. Since then (fingers crossed) things have settled down and I am doing a soak test.
Calibration
I started to look at calibrating the Wavetek. It is a closed-box cal system, all done from the front panel. However, the previous owner password protected the cal menu and the eBay seller cannot help.
I noticed that there is an open 2-pin header behind the Li battery on the PCB. I wonder if this is for resetting the instrument back to factory defaults? Unfortunately the service manual is not helpful, only saying to contact the manufacturer. I am reluctant to short this header out without more info.
Any suggestions?
enut11