The only thing I can recommend is to completely disassemble the entire parallel throw mechanism, tediously clean and lube, reassemble and reset it. The internal assembly is most likely the culprit causing you trouble.

I have just done it to my own Royal Ace and it now performs as new. Remove the butt end scalloped plates first in order to access the interior parts, carefully storing each grouping of parts in separate labeled sandwich bags. The nylon roller bushing and cross beam often are misaligned or worn due to normal wear and tear over the decades.

I was afraid to touch anything but I really have to as I want to move everything over to another RA shell I have with less fade on the wrap. I may hit you up for help if I get into trouble!