I would be having a reasonable guess of the torque converter intermittently locking up when it should be solidly locked at that speed. This would be proven if you could put it into 3rd gear at similar engine revs and it does not surge. (the 250-500 rpm difference is the giveawawy on the converter lock-up)

Try a few of the electrical contacts around the transmission. It may not necessarily show up as a fault by the TCM if it only momentary loses contact.

Other possibility could in fact be the torque converter or TCM or splices in looms etc. (IIRC there is one that does throw a code, Axeman knows about it and where/what to do)