Are you sure you are using one database for your NLB environment? Note that SiteSync does not sync configs, so if you have made config changes (e.g. added custom field or else), you have to manually sync your config files across all nodes in your webfarm.
I could advise you to double check your load balancing setup as described in our online article: Sitefinity load balancing
Last, could you also test if this occurs on all browsers or particular ones?
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Sitefinity CMS Ideas&Feedback Portal
and vote to affect the priority of the items