i've recently had a head gasket and timing belt done,now the car seems to lurch slightly between 40 and 50 mph,it only does it once and is usually but always accompanied with a backfire,i also tow a caravan and it is more severe when towing when the engine is under strain,all the ignition parts were replaced a couple of months ago.It did the same thing but a lot worse last week and on inspection discovered the garage had'nt put the air intake pipe on correctly and it had dropped off at the airbox,i thought i'd cured it but evidently not,could the ignition timing or valve timing be slightly out or is it more likely a vacuum leak.it appears to be backfiring into the exhaust.