I dunno if any of the valves were damaged or replaced when they adjusted the clearance. For what it's worth, this stalling issue can happen even before the engine gets very hot. I've left work and had it stall on me after only a thousand feet or so. Speed doesn't seem to affect anything either. When I leave work, I'm on campus and the speed limit is 20. It has stalled on me there and it's stalled on another road when I was going about 55.
I'm gonna guess your valves were too tight. Were the exhaust valves damaged or replaced? Too tight generally means overheated. Just slightly possible that you have a valve 'sticking', that is, hanging up in the valve guide and not closing entirely. Engine gets hot, valve sticks. Cools down for a bit, then it's fine.