Another one would be to check the ign mod on the inner wing passenger side.. Clean up the connections..

I had this problem for 5 months and it ended up being that the earth to the engine was shite..

Brought a earth lead (Like one u get in a amp kit) and earthed the engine directly to the battery. Clean up a connection either end... The leads are usually around a fiver... After i brought/replaced every sensor the cheapest fix of the all sorted the problem..