I have the same keyboard, as well as the same problem (just happened 3 times as I typed that first sentence). The spacebar on that keyboard is capricious. The problem persists across programs, and is not limited to Word. It could be me (us), but I have not encountered this problem in my other boards.
One reasonable partial solution: I have a Mac and I lowered the 'key repeat rate' toggle, and that helps a bit, but since lowering it seems to cause (for unknown reasons) other issues in the Mac system, I usually leave it pretty high. Frustrated a few months back, I switched to my Model M for a few months, but after brown cherries, typing on the Model M is more of a workout than I could tolerate. So, I'm back to the Filco...and extra spaces.