you will need to find a commonality for when this happens and when it does not. different carrier in use? perhaps you should consider making the closer another agent on your vicidial system and transfer the call within vicidial (transfer to a local closer instead of making a 3-way call). this allows the closer to have the client data on their screen. if the closers do not have computers, and therefor cannot log in, consider using them as Remote Agents in a closer campaign which will still allow transfer to closer (which is still not a 3-way call). in both cases you gain the ability to see the call in real time during closing and statistics regarding closer talk time, etc. Then the transfer would be "consultative" at first to allow the fronter to discuss with the closer before leaving the 3-way call.
you could also consider using the stock vicidial interface instead of the goauto interface to see if this makes a difference (entirely possible). you could also consider upgrading to a later version of Vicidial which may resolve your issue. you'll lose the look of the goautodial screens, but you'll gain new features and a more thoroughly tested distro.
do try to find a commonality to the occurrences, though. be it server load, area code, carrier, or even time of day. something to be able to reproduce the error so you can more easily troubleshoot it. you can obviously try troubleshooting it now in /var/log/astguiclient and /var/log/asterisk ... but it is a lot of reading.