Hi Heronfan, Long time no chat!
The timing can be thrown off by a few things. I too have an old JCPenny workhorse, circa 1978 or so. I'm not sure if the hook is driven by gears or a belt on those machines. I have long since retired mine and only use it as an emergency backup. If the belt is going that could be causing your machine to go out of time more often like you describe. Then the belt should have been replaced. You can adjust the timing on a machine without replacing a drive belt. In fact most often that is the servicing they will do. They usually don't replace the drive belt unless there is obvious wear or it completely stretched out of shape. I know on my machine there was a drive belt that ran the needle bar (needle going up and down) and there were adjusting gears that could be moved out to compensate for a belt that had stretched out of shape from use to put the tension back on it. It is possible the same goes for the hook (aka bobbin race) and perhaps your last two servicing they simply adjusted those gears that the belt runs on. It may be time for a new belt if yours is similar to mine.
Other things that can cause it, if the needle hits a presser foot or needle guard or you try to stitch through several layers of heavy fabric that could do it too, especially if your belt is worn.