engine runs perfect when in park and can be revved up, but as soon as a load is put on the engine it starts to mess around. travelling along the road it will sometimes run ok but then when acceleration occurs it will missfire until a cruising speed is reached and the throttle is eased off and then it runs ok. it only ever seems to be one cylinder. it's easy to determine this because the one that is misfiring is also the one that has a small leak from exhaust gasket which has been there for almost a year and ran perfect before...........

 

You failed to mention much about the car, engine type, mileage, age.. In tha absence of anything specific, one might guess that there could be a bad spark plug in that cylinder, or the plug wire for that cylinder is getting bad. How long has it ben since the last tune-up?
You mentioned a "small leak from the exhaust gasket". Leaks like that do not tend to fix themselves, more likely, it will get worse with time. It shouldn't cause a misfire since it's outside the cylinder if it's at the exhaust gasket.

 

oops.... its a voyager le 3.3 petrol. i mentioned the exhaust because by pure coincidence ( i think) when it runs only on five , the blowing stops...
it was about 6 months ago but to be honest can we really trust some big name garages ...... no offence meant to any honest mechanics by the way.

 

its also gote 179,000 on the clock.. couldn't understand it happen only when engine under load... in park you can rev nuts off it......... not for too long of course