I used to have this problem often. The solution is a bit weird but works.

Delete any other saved file or game over 2 gigs in Size.

Not sure why, I could not isolate the issue, but if you have too many downloaded games too large in size, it will freeze up. Since doing this I have not had a single freeze in over a year.

That's one of the two possibilities I thought of, however I am using the same PS3 and hard drive I was in December when I was getting these all the time, and I didn't have any of these for months.

The other thing I think it might be is RAM. The fact that so many people report this happening just as they're firing or even overheating burst HMGs makes me think this is more likely. Basically when there's so much going on on screen and in physics calculations (etc) there's too much to hold in RAM and the system crashes. There's typically a lot going on when your burst HMG overheats. I am FAR from an expert but for me it does always happen in high intensity moments, e.g. when ADSing at a group of enemies who are all firing back, or when changing direction rapidly in an LAV. The frozen frame always looks action packed, like a screenshot they'd give to IGN for publicity. Never had it when I'm just walking through a field on my own.