Car will start, barely, then run for just a few seconds then die?

The belt is fine, I was thinking fuel but im not a mechanic by any means. As far as tune up, engine oil, air filter spark plugs are all good. This is my girlfriends car and when we first met her car was having a motor changed out on it, not a new motor but a 'working' one was being put in it. When we went to pick it up the shop was needless to say "shady" so I didnt trust that to begin with. The car has not ran great since we picked it up but for the last week she said it was making weird noises, she was on her way to work last weekend and about a mile down the road she called and said her car had died at a red light, managed to get it started back up but the car wouldnt go over 30 mph, told her to just bring it back to the house till I could get someone to look at it. Then the next day she called me as I was on my way home from work she said she took her car to the store and I would need to come pick her up because her car wouldnt start. Had to pull it to the house and it has been sitting here since.