I have a 690 with the plunge base and have never had the problem you describe. I use it as a hand router and it has never been upside-down in a table. If it was used in a table then the dust issue could still be the problem.
I do have 3 1/2 HP Sears router in my table and it gets dust in it from being inverted and it has similar problems and needs to be cleaned out. For a while it was like every other month until I plugged up the tube holes with corks now it is about one a year. Taking the springs out sure made it nicer to use. Thanks Router Workshop!
BTW (by the way) a coat of paste wax works wonders on slides, tabletops, etc. I try to get carnauba and put on a couple of coats. Things slide and don’t rust when waxed.