Sounds like you crossed the two ignition wires up. It's also possible that the gap is incorrect on one of the plugs causing a misfire.

sounds like a dropped cylinder. Mine done this when a spark plug came loose... just outside newcastle (130 odd miles away from home :( ) with no tool kit to get the torx screws out to get to the plug... 40-50 mph all the way home anyone? :P

u cud have got 2 leads the wrong way round? so it aint firing right
check manual for the right sequence.
and have u tightened up the sparks properly? and not cross threded em?

someone must be able to help

i changed the spark plugs today but when i started it back up it sounds and performs like a tractor......any ideas what ive done wrong.....im sure i put the leads back in the right cylinders....could i of damaged the leads or sumthin??? anybody have any ideas plz