The wipers on my Suburban have been erratic for a while. The common issue across the GMT400 platform seems to be the "pulse board", i.e., the circuit board with a plug-in on it that lives inside the wiper-motor housing.
I bought a new one locally for $23. Opened up the housing, pulled the old one out to replace it. There was purple grease all over the contacts and the circular gizmo that the contacts touch. I decided to clean it up and see if maybe that was causing the malfunctions. Wiped her clean, and sure enough, wipers working perfectly. Took the part back unopened.
Just thought I'd put that out there for anyone having wiper malfunctions on their GMT400. Might try cleaning it up before replacing it. May or may not resolve it for you. I believe the GMT800 platform uses that same board, so it may be an issue with them too.