Okay. So I did a big ol' freakout yesterday. I think what happened is that I did bad research and bad buying, which is embarrassing and pricey but not the end of the world. Basically, the cobs are just a hair over the driver's capacity. When the circuit open, the driver is putting out more than 1400mah. Combined with the temperature staying pretty low and you the cobs trip the over-voltage protection. Should work at ~250w slightly dimmed (that is, with a 100k potentiometer that isn't straight garbage).
Only smart thing I did in my overreaction last night was disconnecting one of the cobs. The circuit now runs 4, and that seems okay. 200w@full power is way more light than I was getting before. Potentiometer is definitely shitty - just kind of jumping around, not really moving smoothly through the range, etc - will replace it with a higher quality part today, as well as rewire everything with higher gauge higher quality wire, and probably replace some of the terminal blocks for wire splices, just to reduce strain. That should allow me to run the 5, unless there's some other problem.