Olds TS125.

[Manky Monkey]

Ha ha!
I had similar symptoms with one of my old Bonnevilles years ago -ride, stop, check everything, ride again, stop again. Turned out to be chrome cleaner blocking the vent hole in the fuel cap!

[the coppersmith]

Oh 

[BikerGran]

And I had a similar problem with my little 2-stroke, had it in the workshop twice and they couldn't find it, in the end it was a mate of my daughters who told me to take it out and ride it till it happened then phone him - he came straight out and checked all the electrical stuff and it was the generator failing when it got hot.  It checked out perfectly alright when cold, which was why no-one could find the fault.

[Olds]

Old chain was somewhat knackered, which is not surprising as the bike has been used almost entirely off road, so time for a new one, plus sprockets naturally. Thought I'd try upping the front sprocket from the std. 14 T to a 16 T as it's no longer used for green lanes (we don't have many here  ) Bike is actually better for it as it's dropped the rpm down to 6000 at 60 mph with no noticeable change in acceleration. Makes keeping up with in traffic less frantic.