Idling Problem

[howard milliken]

i put a small amount of diesel into my petrol focus, got her home and drained all the mixture out of the fuel tank right up to the fuel pump. replaced the fuel filter. filled her up full of petrol, car went alright until the fuel level dropped down to just above the minimum on the fuel guage. The car started to misfire and was idling badly. i changed the spark plugs, ht leads and the coil pack, car went alright until the next time the petrol was low and same thing happened again. can anyone please help me.?