This exact same thing happened to me today! I had this problem intermittently in the past, but it went away. I did not put together when it went away and what I did to my bike.
BUT, last week my pork-and-beans gas tank cracked at the petcock, so I went back to my stock gas tank. Today on the way to Target, I get a severe loss of power! I checked the head bolts since I assumed it was from losing compression (this was the first ride since going to just one head gasket on my Puch head), checked the intake and exhaust mounting nuts, etc. Everything was tight.
On the way home, she ran great! Until about 10 minutes later, loss of power again.
After cruising the forums I saw people suggest the gas tank vent. I popped the gas tank vent, cleaned it off, duct taped the holes on the side, leaving open only the holes on the bottom and the vent hole over the gasket. Blew through the holes on the bottom, nothing.
Instead of getting all fancy, I just drilled a 1/16" hole in the cap. Now she vents like a champ, and I rode around after that and no issue with lost power!
I will come up with a better solution at some point, just glad there was an easy solution to this problem.
