Hesitating, jerking and stalling

My car is a 1992 six cylinder four door with 135,000 miles on it. I had my mother bring my car to an auto repair shop because she works for a local auto parts store. So, I took her car to work while she took mine to the shop. The car was hesitating and jerking about half of the time when I press the gas pedal while driving, especially when I was starting out from a complete stop. The other problem was that the A/C fan would only come on if the fan was at the highest setting. So, the mechanic ended up replacing a vacuum hose to fix the hesitation and jerking, and replaced the blower resistor motor to fix the A/C problem. The A/C works fine now, but the car is still jerking and hesitating, but not as often as before. The thing that really bothers me is that the car occasionally stalls now when coming to a complete stop or starting out from a complete stop. It never stalled before it was brought to the shop. My mother talked to the mechanic again, and he said that I need a mass sir flow Sensor which would cost another $400.00, which I cannot afford right now, and do not know if it is worth it to fix this car or just drive it as is until I save enough money to buy a newer car. He also said that he "thinks" that the computer needs to adjust and reprogram itself because he changed the vacuum hose (which I think is BS). Anyways, my questions are: Does this mechanic not know what he is talking about? What exactly is a mass air flow sensor, and what does it do? Does it sound like I really need one? Is it worth it to fix on such an old car? Should I try to find one in a junkyard? Is it easy to replace myself? I am somewhat mechanically inclined. Anybody's help would be greatly appreciated. Thank you.