Ok, so i have a diamondback response mtb and for some reason the front fork is really loose. i can hold the front brake and wiggle the fork, i dont like it cause i ride it off jumps and stuff, so that could be a problem if it gets worse. i think something is either too small or worn down between the stem and the fork, how should i go about fixing this?

additionally, idk what the name of the piece is, but the circular "nut" that fits to the top of the stem to the fork broke off, what the name of this piece and can i buy a replacement w/out having to buy a new stem or something else?

heres a pic:
http://i49.photobucket.com/albums/f300/crfcryzyzfltz/IMG_5293.jpg


ill see if i can post vid of the wobble later.