Help please!!!

about 2 weeks ago my fiesta 1.25 zetec was mis-firing on idle and when driving, it was due a service so i changed all the usual hoping that it was down to a dodgy plug, when i changed the plugs the plug closest to the drivers side(think thats No. 4) was black, i checked the leads as well and the lead was corroded at both ends and so was the coil pack, i have changed the lot now.It is fine now when idle but when driving it is still occasionally mis-firing, it tends to do it most when reversing, when it hits 3000rpm or when its hot as it doesent happen for the first 20 odd minutes of driving in the morning when its cold. I am reluctant to take it to be checked as i am getting rid of it in january but need it running smooth until then, any ideas?

 

Crank Sensor possibly........?????????