Now, does it do this while you're typing, or does it do this only when you're using the space bar?  It could either be that the escapement mechanism is either loose or out of adjustment.  What I remember about Underwood portables is that whenever you use the space bar, it doesn't space forward until you release it.  Same way when you type--the universal bar frame in the back of the type bars--the same kind of spacing happens.  Take a look under there when you use a key or the space bar and see what the escapement mechanism is doing--whether it is loosey-goosey or if something looks a bit worn and needs to be adjusted.  Let me know what you find out.