ReRap Firmware stepper output dropped

I have a mini kossel running duet electronics (0.6.x) with reprap firmware. Everything was working great for the last couple months when all of a sudden a couple days ago my steppers started missing steps. I've corrected the problem for now by increasing the current to the steppers with M906 from 400 to 1000. My 12v power comes from a 600w atx power supply. It's still delivering 12v (with no load anyway, I don't have a good way to measure under load).

Previously running the steppers at anything above about ~600 in M906 would cause them to overheat from too much current. Now they're cold to the touch at 400 and about 40c after a print at 1000.

Everything seems to be functioning now, but I'd like to know the root cause. Any thoughts?

1. Check that the screws in the 12V power input terminal block are still tight.

2. Can you supply the specification of your stepper motors, or at least the model number? Stepper motors that run hot at 600mA are unusual, but not unknown.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

The steppers are Minebea 17PM-K402-P4V, I believe they're 76oz. I did some googling but couldn't find a spec sheet on Minebea's website, I'm wondering if they're an older model.

I'm looking through my config.g and I think I maybe posted a bit too soon after the new year festivities. I think I upped my max speed and accel via M203 and M566 when I was messing around the other day and forgot about it. I could definitely see that as being the cause for suddenly skipping steps when I wasn't before at the same current levels...