The bottom connection where they screw onto the mast isn't very secure. Over-tightening them exacerbates the problem.
I'm not surprised to hear that--the resonators don't seem terribly sturdy. I'm using either star-type or spring-type lockwashers on many of the connections, to keep them snug on the road. I guess I won't be surprised when some of the resonators fail, from being screwed on and off the mast top. At times I may stay on just one band for a week, or I may change bands a couple of times in one day, depending on how propagation is.
Luckily, I've got a number of spare Hustler resonators & whips, because they've been reasonably inexpensive and I've been experimenting a lot. I've also found a few used for cheap at hamfests, since so many people are using screwdrivers now. :-) So I've got a good stockpile to play with.
I have played a bit with with bigger coils ( there's one visible in the lawnmower page, http://www.duke.edu/~kuzen001/lawnmower.htm
), and I think some day I'll get back to experimenting with those and try to replace the Hustler resonators with a homebrewed multiband bugcatcher.
Either way, I sure am having a world of fun from the car, with the high bands coming alive again! :-) (And from the lawnmower, too!) ;-)