[Boatanchors] Drift with NC-303

[w3khg]

Hello

While listening to cw signals this morning on the NC-303 the signal in 
the passband slowly drifst out of the passband which required tuning to 
center it. Other signals could be heard with the same occurring. Most of 
the time it's a fast drift and then the signals would jump 50 hz or 
more. Sometimes they would drift back into the passband. The receiver 
had been turned on for 20 minutes prior to listening. This is not the 
first display of such activity. Most of the receiving time it doesn't 
exhibit this behavior.

Any idea's on what may be causing this and where to start for 
troubleshooting?

thanks

john/w3khg