Hi Conrad
If you have an android device that support dual radios (latest nexus 7 tablet is an example) you can install an inexpensive app to show the presence of 2.4 and 5gHz at the venue. Without the ability to see what else is operating at the time the interference occurs the ability to troubleshoot will be limited. As it worked fine prior to 6pm it would indicate that something was turned on at that time or that there were too many 2.4 devices present in the frequency band. Can the line6 devices operate at 5 gHz? That would be less crowded (for now) but a wi- fi scanner or app will show you what is really going on.