Every time you heat cycle a set of tires you take some life out of them. Take a look at the karts on the grid for qualifying. Most will have a new set of tires. Mid compound tires seem to be the worst. Take a Bridgestone YHC for instance. They are done after one day in my opinion. If you kart is consistent in the first several laps, then goes off, sounds like too much initial air pressure over heating them.