My brother's bike has been cutting out on him lately. This bike was purchased used, and has been a multi-layered lasagna of headaches.

This latest headache happens when rolling off the throttle for a downshift. It isn't every time, or even every day, but frequent enough to annoy me.

I have no clue what his latest mods are, but I'm thinking vacuum leak somewhere. It's not my bike, so I'm not going to go snooping around. I'm just looking for a "hey, have you checked the...?"

GM

 

I had that happen and just for grins I changed the system relay, worked.

 

Have you checked for:
intake and exhaust leaks
dirty IAC
loose battery and ground cables
pinhole inside the tank's fuel lines
dirty fuel filter
check to see if the bike has thrown any codes

That should be a good start for now