I appreciate you may have tried the following:
Harrow Technical used to service them but their website now mentions they don't service 6x7s anymore. Alternatively Asahi Photo
http://www.asahiphoto.co.uk do Pentax repairs and mention 6x7 servicing on their website. In the US there's the legendary 'Eric' - but I don't know how you go about customs when shipping to the US for repair.
The cameras are hard to find in the UK (being snapped up quickly from dealers when they list them) so repairs are likely to be cost effective as well as restoring your camera back to fully functional.
Again you may have done this: I'd double check your film loading technique or try a roll of Fuji that has the 'peg' in the take-up spool slot which may help?
Please keep us informed if you find somewhere to service it.
John.