My 2000 chevy venture when is running maybe over 40 mph the iddle starts going down and up, why?

Could be several things, like dirty air filter, fouled spark plugs, bad gas, bad fuel pump, clogged fuel filter (probably located on the pump which is most likely located inside the fuel tank.) bad EGR valve, a vacuum leak... the list goes on and on. but most of these would cause a trouble code or service engine soon light to illuminate. if that light is on then you might want to get a parts store to read the codes for you and then repost with the results. good luckl