at what speed/rpm in 5th gear?
as arcangel said,, seems to indicate fuel or air restriction,,, possibly a case of sumping,,,, check air filter,,, remove in line fuel filter if it has one,, never run inline filters... check vacuum petcock,,,
if all this is good,, to an experienced eye,, remove the petcock and check its filter.. check tank vent system,, if insufficient air is introduced for fuel used,, ir will do as you say yours does.. check crossover line..
My rule is perform a cold compression test whether I think it's required or not, since that gives me a basic idea of top end health. We did that with every make and model that came into the shop and it pays off.
Also agree with the fuel checks above. I've run inline filters since the early 1980s but I don't run tiny ones and generally use angle head plastic automotive filters. Preferences vary but the vacuum fuel valve is one of the first parts I throw away and replace with a Pingel. (I've only had one Pingel seep after 30 years and a rebuild kit fixed that.)
Yah I stupidly thought coating the inside of the tank I had made was a great idea with creme sealer..... Not so much. 3 years later it failed and had and still have some debris left in there. But since then I changed to high flow Pingle with screen sock and I always run the glass filters just as an extra. Yes like Terry said a good point check venting witch I believe is your fuel cap on your bike. Run the cap a touch loose and see if that fixes it to tell if that's a problem. I changed to a nicer cap once and didn't realize it wasn't a vented one and had issue till I thought about it...
like casement said,, i performed compression tests, on all bikes that were in for any service work.. and DTC codes on later bikes.. you can never have too much info...i even made a copy of all tuning maps, and entered it into the bikes owners files..