Call to undefined method GFSqlSrvSync::lead_detail_long

We have been getting many complaints from our visitors, where they all report the same error:
Fatal error: Call to undefined method GFSqlSrvSync::lead_detail_long() in /[...removed...]/wp-content/plugins/gfsss/gfsss.php on line 1238

It is not happening most of the time, and we haven't been able to replicate this error with any consistency. We have even tried filling out the form with the same exact fields as the visitor who got the error, and it will work. We have tried duplicating the form and tying the page to the new form and we still get the complaints.

We are using Gravity Forms v1.6.11, Wordpress v3.5, mySQL v5.0.95, PHP v5.3.19.
Here is the form:
[...removed...]

Any assistance you can provide is very much appreciated. Thank you.

Yes. Unfortunately, there is a custom plugin created by the previous developer on this site. It looks like the only purpose of the plugin was to store the GF tables on a MSSQL db that is separate from the normal mySQL db that houses the rest of the Wordpress. I'm not sure why.

I was really hoping it wasn't an issue with this custom plugin, because I know you can't help with that. Is there anything about this error that looks familiar?

I'll have to figure out why they felt the need to setup this custom plugin in the first place. Hopefully it is not necessary and we can disable it. If it is, I suppose I'll have to update it.

Thank you for your time. Feel free to delete this topic.