Okay, now what the hell is this?
I’m going down the home stretch, have the hot-end wired up (but may need to revisit…High Temp Copper gasket maker may not be the right fixative to hold the thermistor…luckily I have four others.
Heat’s being generated (and measured), the extruder is extruding…I throw my first full g-code at it…and it’s pulsing like mad:
Of note…the pulsing is identical to the pulsing of the activity light for the hot end power.
Bridge the polyfuses (big yellow things on your control board) and see if that helps.
Humph, I thought those were caps…what’s the negative effect of bridging fuses? I’d hate to pull a safety feature out of the system, expecially if the problem is odd power…
Well, obviously your electronics won’t have a fuse to protect anything anymore (except for the PSU’s overcurrent protection, which generally works really well in ATX supplies). But try it for now, and if the fuses were really the culprit, you can always add an automotive fuse in the power lines.
I’ve had too many problems with those fuses (and have seen other folks struggle with them as well), so i’d generally recommend getting rid of them and using a fuse that has two states - working and irreversibly blown - instead of a polyfuse with three - working, borderline working, and tripped.
Also, have you taken a multimeter to the PSU and checked if the voltage stays at 12V?
I’m going with bad power supply. (last power cycle, the screen was garbled…current cycle it doesn’t power up at all) I’ll get a known good one and retest.
Same crap is happening to me with my extruder. trying a new power supply now!
I’m starting to think RUMBA boards are a big pile-o’-crap… :[
I find every time I make an evaluative statement like that, it ends up being operator error. 
@Mike_Miller , I have experience with the RUMBA board, and I assumed operator (me) error, but as time goes on, I’m finding a lot of people having the issue with getting the board to program, etc.
Whelp, the polyfuses aren’t much of a fuse. I screwed up, didn’t check my work, and applied 5V across the 12 v circuit. Let out the magic smoke (evidently 5v and a WHOLE LOTTA AMPS).
(See my next posrt. 
)