Re: i'm stumped !!!
Had a similar problem some years ago with a Harley, no warning lights or cat convertors, fuel injection to complicate matters, and I appreciate that bikes are not the same as cars. Bike would cut out, run on one cylinder almost die and refuse to pull, at random speeds and intervals, took six months to diagnose (bike just happened to do it while it was in a workshop for another job) carb was checked and rebuilt several times, ignition changed from electronic to points and back, plugs and coils changed loads of times, manifold seals checked, turned out that it was an intermittent short circuit (to the crankcase) from the alternator. Whenever it happened the voltage available to the ignition coil dropped so much that there was little or no spark. By coincidence I had almost exactly the same problem on a Chinese bike when it was almost new, because I had the experience with the Harley I knew what to look for first, as the stator (the bit with the coil windings) warmed up the insulation on the wiring broke down and the ignition coil voltage collapsed, engine cooled down ran normally, warmed up power fell off then died. Ran a multimeter check on the stator output and waited till the engine warmed up and watched the output drop from 13.5 to nearly zilch. Might be worth a look.