I've been following this with interest and now have to put my 2 cents in. If the bike runs good for 20 minutes before it starts acting up, you either have a heat issue like an overheating coil or, like Headtube suggested, your tank isn't venting to atmosphere. When it starts running crappy, open the fuel cap. If the tank develops a vacuum, you won't get fuel to the carbs. If you can hear the air rush in when you open the cap, it isn't venting. I think I saw you have new coils, so my guess (only a guess since I'm not there) is tank venting.
Don't worry with the collective wisdom here, you WILL get this sorted.
"There’s probably no better way to go from New York to San Francisco on a motorcycle than on an R90S."
, January 1977 and Mike last night