troubleshooting video

I have an IRIS that I've been working on off-and-on for a while. I'm currently looking into the graphics circuitry. For a while, when I had it connected to the SGI monitor it displayed a garbled version of what I assume was the PROM monitor (pixels/lines were skewed across the screen back-and-forth). I assumed that it was a video mode problem, checked the switches, and put it back on - nothing. Then I tried a Sun multisync, and it switched into its "I don't know what to make of this input signal" flashing-light mode. Checked switches again, tried with BPRN and BPRO jumpers in, tried again with BPRN/BPRO back out. Put the scope on the IRIS sync output, compared it with a Sun 3/110 CG4 output. (thought it might be a sync issue) The most common regular waveform (I assume it's HSYNC) is ~28KHz on the IRIS and 61.72 KHz on the Sun (measured with the DSO's cursor function - digital scopes are trancendentally cool!) The Sun's sync is similar to published for 1152x900, the SGIs is not quite what was expected for "Mode 1" on the IP2 (for either primary monitor or secondary monitor back-panel setting) Primary is supposed to be 60Hz non-interlaced, secondary should be 33Hz interlaced (no HSYNC given)