Any chance you forgot to shift? Sounds like the rev limiter is kicking in and the speed is right for that.
I'd start by closing the petcock and shutting the engine down while it is acting up, then check the contents of the float bowls to see if there is a problem with fuel flow.
Next, since it's easiest to do, make sure the idle screws are both tuned for best idle speed. At around 85 - 90, the idle jets come back into action as a power enrichment circuit, so this could possibly be the problem.
If you can tell which cylinder is cutting out, try swapping the plug wires to see if it follows the wire. If so, the problem could be a weak coil or the timing sensor gap. If it doesn't, swap the sparkplugs from side to side to see if one plug is breaking down.
A long shot is a floating valve caused by a weak spring or sticky guide.
