Ok guys, update: I had tried to call the shop I bought it from (CWF in Cineleisure) to explain the problem. The shopkeeper mentioned that he has not heard of any problems with HD DVD or Blu-ray discs up until that point. He also queried whether I was running it on a player or the 360. I described the full problem to him, and asked whether the store has any return / exchange policies. Understandably, he told me that because such an issue never surfaced before, he'll need to check with his supplier whether anything can be done. I told him that I would lend the disc to a friend for further testing (this was before I posted my problem).
The test results were no better on my friend's 360, so I called the shop again on Saturday morning (after making my first post). The other shopkeeper picked up; I explained my problem to him once again. He suggested that I bring over the disc to the store for checking. I also left him my name, which he recognized. He also made sure to relay my message to his colleague. Fast forward to the evening, I walk in the store; the shopkeeper sees me; immediately brings me to the back and hands me a new copy of 300. For some reason, this surprised me very much, since I have relatively low expectations on how they can help me. Maybe I'm too used to being screwed over product returns?
On to the new disc: I found no such problems as my first disc. I jumped between scenes like monkey on crack, sometimes skipping tracks, sometimes fast forwarding, and sometimes by using the scene selection menu. I'm happy to report that no halting or jerking occurred

Though their prices are not the best, I'll definitely get more titles from the store in the future since they've earned my trust and respect.