Unfortunately I have the same problem with Simutrans 102.2.2 and Ubuntu 10.10 X64. I use Simutrans under Windows 7 X64 too, but there I haven't problems.
It is an old problem started some months ago, with Ubuntu 10.04 (10.04 and 10.10 have the same version of Simutrans in their repositories). Looking on Google I found that it was reported on Launchpad in March.
https://bugs.launchpad.net/ubuntu/+source/simutrans/+bug/537803 (https://bugs.launchpad.net/ubuntu/+source/simutrans/+bug/537803)
It's so depressing to not see any changes in these 9 months... Nightly versions of Simutrans 102.3 have the same problem?
I have split this post and moved it to the general "Development and bug reports" section, as it is not specific to Simutrans-Experimental.
The usual freeze comes from broken sdl_mixer lib with 64 bit and certain drivers. Start simutrans with -no_midi should fix this.