power loss

Machine was running perfectly, then lost power but still ran, got it home. Tried to look for problem, thought it was spark, fuel, then gave up! Took it to shop, they replaced plugs and some valve? Ran fine, packed it away for season! The other day, starts first pull, take it for a spin, running...

I have a 1999 Vmax 600 twin. It starts every time 1-2 pulls and runs great. It'll rip like a brand new sled for the first 30 minutes of riding. After that, I notice some power loss, maybe 10-15% less. Around 10-20 minutes later, it won't rev about 6k, and takes up to 20-30 seconds just to get to...