Ads, it's awful seeing you running in circles with this issue. After all the work you have done, and having the bike running well - unfortunately not for very long - I really can't see any need to check the valve gaps and cam chain tension
again. These are not things that go wrong in the short term. Likewise, you've stripped and cleaned the carbs and reset the float level
again, and unless you've got something consistently wrong, which I doubt, I don't think that's where the problem lies. So I'd be thinking about the electrics. OK, outside possibility of a fuel supply issue, but the symptoms don't sound like that. For example, inadequate flow from the tap would show up on giving throttle and blocked tap vent would show up after twenty miles, but not after one mile . . .
Summary - I agree with
@atom4488 and would look for something in the ignition; triggering the spark, strength of the signal going to igniter box, igniter box itself. Perhaps go through the ignition system - ignition switch, wiring to Kill switch, thence to coil, igniter, sender. Check for loss of voltage along the way. But the the way the problem crops up maybe says dodgy switch or dodgy connection somewhere?
Intermittent faults can cause grown men to swear. BTW that's a joke.