Well,

It happens. I had a Viojett impellor do something inside the airplane such that when I landed, the rotor had no blades on it. If it happens more than once (not to mention twice) in the same airframe, it makes me wonder if something inside there has changed. I wonder if when you tighten (or loosen up by use) the fan shroud to its mountings, is the shroud somehow being twisted or something? Just a thought. Good luck.

And, yeah, as BVMJethead said, maybe you need a turbine......ASSUMING, OF COURSE, THAT YOU DON'T ALREADY HAVE SEVERAL and were just looking for thoughts on the problem with the DF that you also happen to own...