TKS98GT

So i was wondering if anyone could help me out. I bought my 98 GT 2 years ago and it has ran really well and only has 70,000 miles on the tranny...but sometimes when i put the car in reverse it pops out of gear and sometimes when i try to shift into third from second it doesnt want to go in and i have to try a few times to get it to go in...the tranny place wont even look at it to tell me what the hell is wrong for under $300 so i was wondering if anyone could help me out..