Im new to lugers too, but have been around guns for along time. And there are a few unwritten rules when fixing a problem with a highly valuble artifact. First do what is easiest and cheapest first.
To me it sounds like its probably been since the war since its been stripped, cleaned, and oiled. Thats the way my grand dads 1911 from wwii was. I say it probably needs a complete stip and clean but that might be over kill, i just like to make sure they are nice a clean. (Some say i have OCD with my guns
) It sounds like something is gunned up, but if its mechanical then its got to do with the weak spring. (not completely familar with luger parts yet or their names.)
I would Carefuly take it apart. There are vids on youtube under luger that will greatly help in taking them apart. Follow them closely and dont get ahead of him, also be very gentle with the parts of your luger, they are old and have had a long life. (No offence anyone)
Jarrod M