try disconnecting the cold start multiplug then see how it starts. If theres no difference then take one of the pipes off the aux air valve and blow down it, if you cant then that means the valve is knackered (it should be fully open when cold). If thats ok then check the wiring to the electro magnetic pressure actuator on the side of the metering head, I had a very similair problem to you and it turned out the wires going to this were badly corroded.

Duncan