2000 Mustang gears sticking

I have a 2000 Mustang, 6 cyl, manual with almost 100K on it. My gears stick, as in, if I sit in one gear for too long it won't let me out of that gear, or if I sit in nuetral for too long it won't let me get into any gear. If I get stuck I have to turn off my car, put it in first and then turn it back on, so that I can go. It will also get stuck in reverse, or not even let me go into reverse. What's the problem?