Luger Jamming

I got my Luger (I asked some questions about it a few weeks ago) and it turned out to be (according to the Luger handbook) a 1920's military police with 95% bluing in 'good to excellent condition. I took it to the range, and, as a poster said below, after some oooing and awwing and pushing back the people salvating over it, I loaded it up (new mags) and started to fire. Well, the first 3 rounds went ok then..jam. The bolt slides down over the bullet and shoves it up and almost out of the receiver. Like it's not giving the bullet enough time to come up. It did this a few times, someone who knew about Lugers told me they do this a lot. I said Ok and went on firing. I went back the next day and the clip I used did this every shot. I switched to the other clip.put through 15 rounds, no jams. I was using 115 grain Winchester FMJ (yes, I know not to use hollow point) 9mm Luger rounds. I read in your forums archives to use 124 grain. I'll try that (Winchester makes it). Do Lugers naturally jam a lot or is mine perhaps somewhat broken, or is my clip bad? I have 90 days of free gun smithing ( I told you I bought it a great shop) if there's a problem. What a good ammo to use? I'll use the one good clip (the spring seems tighter on it) and that one seems to have no problems. Any help would be great. Thanks.