You can limit the number of rows that should be exported in phpMyAdmin. Don't export the post table at once, only choose f.e. 1000 rows. You can try bigger values, until the script times out.
Donate to UBBDev today to help aid in Operational, Server and Script Maintenance, and Development costs.
Please also see our parent organization VNC Web Services if you're in the need of a new UBB.threads Install or Upgrade, Site/Server Migrations, or Security and Coding Services.