***FOUND A FIX FOR THIS*****
I've been having this same problem with my Euro version - The Milestone.
Went to motorola support who advised me to send it to the repair centre and go through all the inconvenience of not having a phone for a couple of weeks.
So, just before I stuck it in a jiffy bag to send it off I used a bit of initiative...
I sprayed a bit of WD40 into the 3.5 jack, and worked the headphones in and out and around for 5-10 minutes to make sure all the connectors got a good coat of the magic lube.
Works perfectly now - no crackling, no music cutting off, even when I'm tugging on the headphones and therfore... No need to repair. Can't believe this hasn't been tried before in all the threads I've read of the issue.
Might be worth giving this a try before you send it off, seems to work on the milestone variety of the phone just peachy.