I started this morning printing Yoda-lite. It started fine - running at F15000 (250mm/sec in Slic3r), going around like a madman, and the first 10mm printed just fine. (Edit - 0.08mm layer height, so the 250mm/sec is reasonable)
I have a recently rolled Marlin from Build-me-Marlin (built today).
I'm using Printrun, and 250000 baud.
Then all of a sudden, in the middle of the print, it started jerking while doing the profiles, but still going nice and fast on the infill. I thought maybe it was because I closed Firefox on the PC. So I stopped the print, restarted the computer, unplugged the USB cable to the Ultimaker, unplugged the power to the Ultimaker. I let it cool for about 1/2 hour, then powered the computer back on, powered the Ultimaker back on, plugged the USB cable back in, started Printrun again, loaded up the exact same gcode file and started printing again from scratch (after removing the partial print from the table).
But it is still jerking on the profiles, starting on layer 2 (layer one prints at 30% of top speed and prints just fine). Remember, I was able to print 10mm before - about 125 layers.
I thought it may have been the steppers or stepper drivers overheating, thus I let it cool for 1/2 hour. I thought something on Windows 7 might have been interfering with the USB->Serial port driver etc, thus the complete restart. I also reflashed the firmware, just in case. Nothing has helped.
It still seems to follow the correct path - I don't think steps are being missed - its just coming out blobby because of all the pauses, but other than that, all seems well.
Any ideas what may be going wrong? I'm at a loss of even how to diagnose it - are there any diagnostic M-codes that will tell me things like if the stepper drivers have been overheating or anything like that? Is it possible that although the USB->Serial interface is connecting at 250000 baud, it has been secretly slowed down by Windows7?