82 GS300l, no power from 3rd up
 

I have an 82 GS300L which i acquired about a year ago. The bike wasnt ran in a couple years before i purchased it. I cleaned the carbs, replaced fuel and battery and it started right up. The problem im having is that when i get up into 3rd and up, its bogging down and there isnt any power. I had it into a shop cuz i couldnt figure it out and they cleaned the carbs again and it ran fine. Rode it home, no problems. Couple days later, I go to ride it, same thing starts happening. No power again. Took it back in again and they think it might be electrical. Comes out of the shop, runs for a day then same issues. I really dont want to take it back to the shop if i can fix it myself. Any ideas would be appreciated. Thanks.