long story short, my DK came with a 4-board set, and I performed the works on the monitor with deluxe cap kit and got the B+ adjusted spot-on at 108.0V and it had this continuous wave in the picture (I think most describe this as the "flag wave"). I'd bought a new voltage regulator (that small IC) but lost it before I could install it.
I converted the wiring harness to an edge connector type and installed this 2-board set that I got with the machine. guess what? the wave went away. it wasn't anything wrong with the monitor after all.
to my understanding, those 2 adjustment pots on the 4-board set can cause this problem, among other things if they're not adjusted right or they're broken (they break easy, I guess). considering the wave went from side to side I'm guessing it's a vertical adjustment (seeing as how the monitor's mounted sideways).
alternatively during my conversion process, I got away from a standard switching PSU conversion and installed a stock Nintendo one. (the PP-7B or whatever the model # is) with the 4-board set running with the non-Nintendo PSU, it would lock up constantly throughout the day. the board had a lot of corroded ICs on it, but I sanded all of those to at least even get the damn game running ... so my problems were power-related.
if you have an original Nintendo PSU, I would encourage you meter one of the roms for +5 voltage to see if it's running too high or low. I wanna say mine was at 5.10V when I metered a rom. if you don't know how to do this, look for the notch in the chip ... orient that notch in your brain as if it were facing the left side, meter the pins in the immediate lower right corner and the upper left. that's how you see what your voltage AT THE BOARD is looking like.
if your ICs haven't been cleaned, which they should have if you had the board serviced, I would encourage you start popping any socketed chips out one by one and inspecting them for corrosion and sanding the chip legs as necessary.
that's about all I have to contribute to this... I've had a horseshoe up my ass with this Nintendo stuff, honestly.
