I know that in a thread in a different forum that Skuzzy said, for best gaming experience in the MA, turn on Port Forwarding for your router, and allow ports 2000 - 5000 to be opened on both UDP and TCP. (
http://www.hitechcreations.com/forums/showthread.php?s=&threadid=68316 )
The other thing, the Jet froze for me back in some older days about 6 months back. Mostly because of the high speed and all the trees around. My memory wasn't able to process the jet's speed along with all the upcoming scenery. And I know MA maps have A LOT OF TREES!
Although your system's memory seems to be very high, the only other reason I can think of, is your internet connection. That it actually generates the freeze somewhere along the ISP route.
But, first of all, try Forwarding Ports 2000 - 5000 on both UDP and TCP to your system. That way traffic on those ports can reach your computer without being blocked.
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EDIT: By the way, just thought of something, do you perhaps have an Anti-Virus and an Anti-Spyware service on your computer? Viruses along with spyware/malware can generate pop-ups and freeze your computer for an un-disclosed amount of time.