I admit that I have limited time behind Glocks, but I have never quite had this problem before with the two other models I've experience with.

Observations:

If the guide rod is not inserted in exactly the right position, then the spring will fold out as the slide goes back, causing an aweful scraping and clicking noise, eventually leading to the dislocation of the captured spring unit.

Mind you, I seat the rod properly between the slide and the barrel cuff, but it would seem that the "ripples" in the spring snag the slide retention system and pop out the peice.

Am I looking at an old model/ bad spring or is this something else?

Are you sure it's in there right? Photo?

Sounds like some kind of aftermarket guiderod.

I'm a big proponent of leaving Glocks 'as is'. I see too many aftermarket parts fail under harsh conditions, where stock Glocks just run and run.

On the G20's, the plastic polymer guide rod should be replaced with a Wolff SS guide rod and a 20-22# recoil spring depending on the loads you plan to shoot.
As you noticed, it must be installed correctly on the slide.
The polymer rods often chip on the end after a few hundred rounds.

Rod: Polymer; unknown compound
Rod #: 5600

Insertion Method: Barrel is insterted into slide; Small end of rod is placed in circular opening underneith barrel opening at end of slide; two primary notches are present on barrel shaft, underneith chamber- a further out polygonal catch and a closer semi-circle catch which sits deeper and closer to the actual barreled chamber. The flat, large side of the rod is firmly seated down into the latter; slide is reinserted onto the frame.