If it really is valve float, shouldn't matter what gear its in, it should happen in all gears, even in nuetral. Try reving the car in neutral, and see if it cuts out.

Sounds like it is fuel related (fuel pump, plugged filter, bad pressure regulator. Or maybe even a CAT that is starting to plug up.

You can check your CAT by using a vaccuum gage on the intake. Take readings at idle, 2000, 3000, 4000, 5000 rpm. If the reading drops at higher rpm, CAT or exhaust is plugged/restricted.