stalling problems

[puty72]

1989 ranger 2.9 auto.

I got my truck back from the garage yesterday after getting new heads, cam, lifters, pushrods, headers, trans modulator, 3-4 solenoid, and shift kit installed. when i picked the truck up the mechanic told me he drove it 17 miles and the only problem it had is it did want to shift into 3rd but when he let off the gas it did. He said this is because there could be a little bit of air in the tranny lines after removing the valve body. anyways i went to drive it home and it ran pretty good but once i got to my driveway (8 miles from the shop) i let off the gas to check the mail and noticed my truck was about to stall. I got to the top of the drive and went to back in and it stalled when i went into reverse. i waited a little bit and it did the same thing. then i kept my foot on the gas and put it in reverse and then parked it. the shop was closing in 10 mins after i picked the truck up so now i have to wait till monday. I was curious to know if anyone knew what could be causing this