Intermittant failure to start

I have a B5.5 estate 1.9 TDI 4Motion with an intermittent fault. It sometimes cranks but will not fire. I noticed that in fault condition, the glow plug light does not come on at all when teh ignition key is turned. All other lights seem fine. If I cycle the ignition off and on until the light eventually does come on then extinguish, she fires straight away. It's like something is not initializing. I have a Sealey fault code reader/ service resetting tool which displays no recorded errors after the fact, but today I managed to hook it up during fault condition. It gave code P1635 "Powertrain data bus missing message from A/C control. Static - no indication on display"

This all started a year ago with a single event, and has been fine since until a few weeks ago but is getting fairly frequent now. Could be cold weather related? It happens about once a week now. May be unrelated, but very very occasionally I feel what can only be described as a missfire. Very short lived like she missed a beat then fine again. that happens all year round and again no codes recorded. I cannot see the link between A/C and ignition. Any ideas?

Edit 22/04/09: Problem got progressively worse until one day car would not start at all. Found Relay 219 adjacent to ECU to be at fault. ******** had only sold 2 of these items in many years so clearly a rare failure.