The yellowjersey.org site has a PDF (not text searchable) copy of the
Suntour Accushift Problem Solving manual. It doesn't include my bike's specific 7-speed GPX group but the info is still applicable.
I've been using it the past week or so to resolve some long-standing problems with index shifting my '89 Centurion Ironman with GPX group. Going through the manual it appears the Accushift system was very picky about components. Last year I became so frustrated I removed the GPX rear derailleur and substituted an older Shimano 600 in friction-only mode, since it wouldn't index either. But it ran more smoothly with less drivetrain drag so I stuck with it for almost a year.
However I was determined to make the GPX system work in indexing mode. At the moment I have the indexing problem resolved but it took some experimenting. All the original GPX components are back on the bike, other than the original freewheel, chain (a POS and worn out) and cables/housings.
I swapped chains around until settling on KMC Z51, a SunRace 14-28 freewheel to replace the Suntour 13-24, and replacing the cable and single bit of cable housing between the chainstay and RD. Making the cable housing just a little longer seemed to help ease the loop.
I also had to redo the guide and tension pulleys. They appear identical from the side but the teeth are slightly different, one set squared off, the other slightly tapered. I probably inadvertently swapped the two last year during disassembled cleaning.