I have a 1991 R100 GS with only 1700 miles on the bike and recently the rectifier burned a diode which I understand is a frequent problem with these bikes so I replaced the module and the charging system worked fine. Since I just picked up this bike I have little history other than it was garaged kept and the owner moved to Germany. The problem that I now have is that the bike runs fine on the street but when I get it out on the highway it stalls but restarts immediately after stopping. I looked for a pinched fuel or vacuum line but I found none, has anyone seen this before?

1,700 Miles.....!

You could always go to "Old's Cool" on ADVRider, that is where all the old GS guys hang out.They sure have lots of tricks and there is already a couple threads there discussing similar problems. I had one of them GSs for 17 years, so that probably make me one of them too.....

Anyway for your problem, I'd think fuel supply first.. You need to take the petcocks out and inspect the filters, just to make sure the tank coating is not coming off and plugging up the filters.And the filters may have disintegrated after all those years. How do I know that....

Then you really have to look into the main jets/carb bowls, a few of us have been caught by the "little piece of crap" floating in the bowls and sometimes plugging up the jet. But after 20 years of sitting, those carbs probably need a full "Going over", not particularly hard to do, but still check with them ADVers, they'll definitely save you some money and have all the tricks for a proper tune-up

i have never experienced such problem, it being new to me. but according to me may be its the filter problem of your tank. go to a local garage a check it out.