RCA roundie CTC-16 Horiz lock problem

[ws407c]

This one has me stumped. My daily watcher has been running flawlessly for the past year and a half and now the horizontal will not lock in. I noticed just before this happened , the horizontal lock point drifted slowly over the course of a week until the control is out of range to maintain lock. I did slightly tweak the sinewave slug but did not last. I checked the components in the horiz circuit but cannot seem to find anything out of tolerance including the control. Just curious as to see if someone else has had this problem .