96 silverado vortec 350....i was just wondering why when my truck is idiling it will idle up for maybe 2 seconds then idle back down for 15 seconds and idle back up for a few seconds and so on.....when its normally idiling it idles at 700rpm and then when it idles up it goes to 1000.....what could that be? its annoying when your at a stop light and it starts moving if u aint got your foot on the brake hard enough...

 

Mine is a 06 and at idle it will sometimes surge a 100 rpm or so. I dont notice it in drive. I have wondered the same thing if something is wrong. Yours sounds more serious. It could be just an air flow sensor going bad though.

 

It might be you TPS sensor...Throttle Positioning Sensor. Easy fix if its that and it isnt too expensive. Had high idle prob on one of my cars and that fixed it.