Rpm surge/flutter while driving! Help!
 

Ok so I've got a 97 that has been swapped to a 5 speed. It has started doing this thing that is hard for me to explain, but at rpm 1250 on the dot (if I hold it at 1250 while crusing) it will start jumping! Only in like 2nd or 3rd gear, 4th gear to but not nearly as bad. The truck just starts reving up and down so to speak making the truck just bounce up and down. Really annoying...but now if I power through it it won't do it. But if I am just cruising down a bumpy dirt road or some thing in 2nd or 3rd gear at 1250 rpm it will do it. I have no idea what it could be, where to begin to start looking. I really need help!!! Thanks guys