89 Ford Mustang 5.0 tranny question?

[jwjones91]

I bought my car last Thanksgiving and when i had bought it, it had sat for about 2 years, when i got a new battery it started right up but i knew i was going to have to replace a few things. after doing that i finally got it on the road, but sometimes when im in 3rd gear it jumps out. what is the problem with this. is it something else i can fix or do i need to have a mechanic do it. im pretty good with cars, i've replaced the following on this car on my own since i've owned it: Power Steering Hosing, Water Pump Gasket, Thermostat, Oil Pump, Fan, Oil Change. Im 17 yrs. old and yes I had supervision making sure i was doing the right thing. just someone please tell me whats wrong. and just some addition info the car has 130k miles and its got 4:11 gears