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I'm just tuning in to this discussion, so maybe I missed this, but: On my Mesa 7i96 the Mesa inputs aren't like most digital inputs. They're not self-referenced. They all share a "common" pin, which should be grounded if the inputs are active high, or pulled high if the inputs are active low. Is it possible your 7i76 inputs are the same? Have you done that? If you don't, the your input might not be floating, but its reference is, and could easily lead to erratic/intermittent behavior.

paul

I think the main thing here is that there are little real benefits of higher order
interpolation, at least for the speeds, accelerations, and servo period used on
typical LinuxCNC machines.

For example, using a relatively high performance metal cutting machine with say
100 mm/s cutting speed and 0.5G acceleration, the chord error caused by linear
interpolation at the highest centripetal acceleration and at a 1 ms servo thread,
is only about 0.6 microns.

The error just gets smaller a lower speeds and accelerations.

Daan96 wrote:

The axes are driven by 3Nm Nema 23 stepper motors and an HTD-5M timing belt with a 15-tooth pulley, without any gear reduction. 

That will do 75mm of motion for a single rotation of the motor, and will have plenty of vibrations, but since you already have that set, do give it a try.
Normally for plasma we aim at 20-25mm per motor revolution, and this does require reduction from 1:3 to 1:5 depending on final drive pulley.