My car refuses to shift..sometimes??

Ok, I'll give you a full detailed explanation - First off I bought the car (1995 Pontiac TransAm then when I tried to drive it home it was fairly hard to shift from park. I had it for about 2 months and drove it a max of 5 times then I was going to a friends house and when I tried to leave the car refused to shift from park. The shift on would press all the way in and wasn't hard to push in anymore but the shift lever wouldn't move. Had it towed to a mechanic and he fixed it. After I had it repaired it shifted wonderfully, but then, about 2 weeks later, It does the same thing it did before I had it repaired. I took it back to the same mechanic thinking he would do it for free since it wasn't repaired the first time. When he towed it to his shop he said it shifted fine. I came and got the car, It was fine, like the mechanic said. 3 days later - Same thing, but the car shifted the next day like normal, then it happened again, and it shifted the next day. Any idea??