I am going on 6 days now since upgrading to Mono 3.6 and have not had any issues.
upgrading mono to 3.8 (from 3.2.4) also worked for me, I had the same issues as above, also running my nzbdrone on OSX.
Also seeing this the last 2-3 weeks. Ubuntu 13.10. Mono 3.2.1
Upgraded to 3.6.0, will report back
Checking in, no more crash/lock-ups after Mono 3.6. Thanks all.