2001 Mazda 626 Car stalling problems

About 3 weeks ago I started having problems with my car stalling out. At first, it would only stall when I first started it up and put it in reverse getting out of parking spots then I could start it back up and go forward and it would run fine. It got worse over time and eventually would stall every time I started it up and put it in reverse. And now it is very hard to start the car and then it just stalls out pretty much immediately and then after it stalls a few times in a row it wont start up again. I let it sit in the parking lot for 3 days then was able to start it up and drive it to the repair shop. They then replaced the battery and the spark plugs and wires as well as the PCV valve. When I got it back it drove fine back to my place and then I drove it to class and it was working fine. When I got out of class about an hour and a half later it stalled almost immediately after I started it and repeatedly kept stalling and eventually would not start once again and I had to get it towed back to the repair shop. What could be causing these problems? Im not sure the repair guys really know what it is.