boko

over the last few days we have had trouble with the running of our car,it doesnt happen all the time but when the engine is switched on its very grumbly,when the accelerator is pressed there seems to be a resistance and then suddenly the car will jump 2 or 3 times and seems to clear itself and goes like a rocket.it also seems to do this more when the air con is on. any help or ideas please?year of car 1999