As I've written many times here before, the Parabellum should work perfectly with very mild ammo if it's in proper working order. No need to use hot ammo.
I've had my 9mm pistol's recoil spring changed to a very, very stiff one and it still works fine with mild loads so it's hard to believe the spring would be the culprit in your case.
A thorough cleaning and a good oiling is the first thing to try.
Try to see/feel if something is binding somewhere. Does the lock slide in the receiver effortlessly (when you have stripped the gun and the recoil spring is not involved)?
Is it ejecting properly? Are you sure the lock doesn't go all the way back? Maybe it goes all the way back
too fast and because of this the next round doesn't load properly and instead the lock gets stuck behind the cartridge in the mag?
Or maybe there is a problem with the way the magazine seats into the gun?
Probably the best thing would be to take it to a gunsmith who knows a lot about them.
In terms of internet assistance, you'll find the real experts here:
www.lugerforum.com/