tracker shifting problem

I have a 93 tracker A/T, it has been slow about dropping into reverse ever since I got it but now once in a while when I take off in drive it doesn't shift, soon as I let off the gas and slow down then take off again it will shift. It has only done this 3 times in the last 4 months, but I need to know what to check, anybody have any ideas? I have already checked the fluid.