Stalling

About a month ago, the mechanic replaced the motor, which had a cracked block, with a refurbished motor and block. At the same time they replaced the 2 fuel pumps with 2 new ones. I drove it for 2 weeks and everything was fine. Then it started stalling on me. It happens when I'm driving down the road in 2nd or 3rd gear, sometimes I as I'm turning a corner, or sometimes when I come out of the store to go along, it won't start. After 20-30 minutes it will start up after numerous attempts. The starting motor is also new, and it will turn over, but won't 'catch'. I took it back to the mechanic who replaced the motor and they've had it for 2 weeks but said they can't find anything wrong with it. They say it has never stalled or not started with them. It doesn't happen with just me, but happened when my son drove it, as well. Any ideas about what it could be? It will 'hiccup' and then the engine is dead. If I can tromp my foot on the gas fast enough, the engine will revive and we go along. But it happens more and more often. And yesterday, the mechanic said they had replaced a 'modulator' and to come and pick it up. I didn't get for it until after hours and the truck wouldn't start.? Part of my problem is that the motor came with a 90 day guarantee, so I'm kind of stuck with the mechanic who put it in, right?