id defo say sensor is fucked , this happened on my fiesta rs1800 started misfiring at 2300/2500 rpm , run like a bag of shit , and that was running a brand new bosch unverisal 4 wire sensor took me ages to find out what it was i as i thought the bosch sensor would be last thing wrong . it had only been on the car 6 months before it started to play up and then the car started to cut out , thats when i unplugged it and like you said it drove grate so i went to the scrappys and got 4 ford sensors for £1 , banged one in and run fine ever since
cheers paul