how can i fix my corsa hesitation?

i have a 2000 opel corsa the very last of the corsa b's. its a 1.2 16v and i have put a lot of time and effort into this car but it has developed a hesitation. i replaced the thermostat recently thinkin that a overheatin fault was causing it as it had a tendency to have extreme temperatures. along with this i have changed the coil pack and air flow meter but stil once the temperature reaches just above 90 it starts again. i asked my friend who also had a corsa of similar age and he stated that it sounds like a head gasket problem but again i have checked this with negative results. i had an induction kit on the car but it sucked to much dirt in that i removed it and every since i have suffered this problem. my friend also said that he had a similar problem but fixed his by replacing the air flow meter.