Thanks deMort,
I agree that without a strip it is only a guessing game. I had the full frame out in the garage, running it open an closed using a battery drill and while I could hear the load increasing on the drill as it was closing, there was no obvious tights points. I lubed it all up but made little difference. I didn't have the luxury of time to keep it out for more than a day as I needed to use the car.
Re the new motors, I agree, and that is why I want to get to the bottom of the issue. The internet is full of stories similar to mine, all solved with a new motor, but I believe that is a temporary fix and not solving the cause. People could be unnecessarily spending money on motors – although if one had to pay someone to remove and inspect the frame, fix it and reinstall it, it would probably cost more than a new motor.
I might just take mine out again someday in the future when I have all of the other things on my to-do list crossed off... probably 2025...!