have a p reg 16v and had the same misfire problem and got steadily worse changed the crank sensor bought it from GSF got trade discount so only cost me 40 odd quid.When i took the old one off the cable leading up to the sensor had perished and all three wires were touching not good, anyway replaced with new item and so far done about 70 miles and no problem. I had previously changed both the ecu and fuel pump relays but check the condition of the wires on your sensor around the area it clips to the bottom of the block(about 3 inches from the sensor) and i bet they will be shagged, the unit itself seldom breaks 9 times out of 10 its the wires. Hope this helps and its a bit of a sod of a job if your doing it yourself as you will need to move front engine mount brkt out of the way to get to it