rear quarter lights on convertibles

I have a 1991 convertible XJS and have a problem with the rear quarter lights. Mostly they work fine but sometimes one will stay down and not return. This can last for a few weeks and will fix itself. Currently both windows are down and will not return, the motors seem to work as I can hear them and I do not think the glass has come off the guide as in the past it has sorted itself out. Can any body suggest a cure?? thanks

I had a similar problem on a friend's '86 XJS. When the window went down, it stuck. You could hear the motor trying to run, but it wouldn't turn. I think the only answer is to take out the quarter light and runner as an assembly (you would have to do this wether you plan to repair or replace). It's an awkward, fiddly job, but can be done with a little time and patience. If repairing, remove the motor from the assembly, re-connect temporarily, and check if it runs OK in both directions. Lubricate the assembly (guides/runners, etc.)and refit the motor. Re-install, and don't forget to run the roof up and down completely (i.e. without stopping part way through the up or down process)so that the quarter light travel re-sets itself.