I got my truck back from the dealer two weeks ago. They reflashed the comp. among other things. My truck now sometimes surges up and down when trying to drive at constant speeds betwenn 20-30mph. It happened ten minutes ago on the way to lunch and I watched the RPM gauge as it did it. I was driving 30mph when it started it would go from 1500 to 1000 PM and then back to 1500(1500 is normal for 30mph). It did this about ten times and quite when I gave it a little gas. Any ideas??????