So I'll try to make this a long story short: Threw a rod and blew the engine. My mechanic finds a local european salvage yard and gets a new engine. Swaps it out, and there was a knock. Took advantage of the warranty and got a different motor. This one ran great. But at first, it would start up fine cold, but wouldn't start after it had warmed up. Turns out the mechanic has a (sensor/relay?) switched. Everything was working great! A few days after I had got it back, it started something similar. I would drive it around all morning, then in the afternoon it wouldn't start. Came back to it 45 minutes later and it started. Everything was going OK, until a trip into the city. I was stuck in traffic and all the sudden the gas pedal wasn't doing anything?? Kept pumping and pumping, no acceleration. Eventually it came back. On the way back home, did the same thing but the car would die too. Had to get it towed back to the repair shop. The mechanic tried switching out the computer, but did the same thing. He doesn't do much work on volvo's and he's pretty much exhausted his resources. Said the injector's weren't getting fuel, which is controlled by the computer.

So here are the facts:
+ No problems with car before engine blew out
+ Since the new motor has been put it, it's been having the "fuel" issue

What could this be??

Any help is much appreciated. Thanks.