Well, yes, I think you most likely need an ECT sensor. Check A7 for codes in the combined instrument. Often times you'll find a 122 there even if A2 didn't alert you to a possible problem. Yes, a faulty coolant temp sensor can keep you from starting, but...you said the problem occurs when the engine is warm in a "restart" situation. You also said that you suspected "low" fuel pressure...these two don't go together. If the ECT is telling your ECU that your engine is not warm when it is, the resulting problem would be TOO MUCH fuel, not too little. The engine would flood, and you would be unable to start the car until the plugs dried out.

If you have spark, and you have fuel, you obviously have one of them that is not up to snuff. You have a new "Volvo" pump? You have spark to ALL the plugs? When you pull a plug...is it white tipped (lean) or black (rich)? If you unplug the air mass meter, will the car start? We've had problems with the flapper on the airbox getting stuck shut and heating up the air mass meter.

I'd also check all my body grounds, a weak ground will lead to a hot sensor.

Chris