Portacolor Troubles

[dtuomi]

I have a portacolor, made about 1968 or so. Its original, never having been serviced. Its in use everyday about 12-14 hours a day. it produces a nice picture, I've posted a screen shot elsewhere and can link to it here. The problem I'm having is that its moody. I'll be watching it and every once in a while it will lose vertical hold. Its not all the time. It can go days without problems. Someone elsewhere said to test the vertical hold pot, but it seems to be within spec. The tubes are stable, all checked good. The caps seem to be healthy. I'm at a loss as to what its current problem is, or more particularly what makes it fail. it doesn't seem to be temperature, as its in an air conditioned office, and keeps at a fairly even operating temperature (no over heating and no tube scorch on the PCB). Anyone have any ideas of what to look for next?

David