Why is my 2004 Buick Rendezvous shifting up and down obnoxiously?

Ok, so my transmission is being all sorts of awesome (sarcasm!!). its shifting really hard, up and down, will not go over what im assuming to be 40mph (i cant tell because my speedometer stoped working as well). it just goes into neutral after that. i just bought the dam thing so i'd like to know why...