immobilizer problem

I recently had the head gasket replaced on my 300tdi, since then the alarm/immobilizer has developed an intermmitent fault. can it be bypassed? if so how?

Makes a difference if it's a "fly by wire (98 on) car.
Early car, simply run wire from ign switch to pump solenoid or fit Spider link cable.
Late car, pretty much just the spider over ride or get some help-you have to crack open the security module on the pump body.
Have you checked the starter feed is ok? (The little red and white wire).