I currently use a 1987 1391401 with a SDL cable and Soarer's converter. The keys tend to repeattttttttttt and stickkkkkkkkk when i start typing non stop for a few seconds. The keys do not stick physically though. Not only do the keys stick a lot but the keyboard also jeopardizes my mouse and internet browser. My PC would no longer register any left click, and my browser would hang in a way that I could not do anything to change tab or even quit it.
Unplugging and plugging again will only solve the problem temporarily, and the repeating keys would show up again. Not sure if this has anything to do with the hardware of my Model M, or if it's a defect in the converter.