Hi Mark,
Also check out what scheduled tasks are happening around this time, maybe this is effecting your shop. (for example, is the Clean up carts schedule, backup schedules etc.)
Also maybe its got something to do with your skin templates for the site, if it works in firefox and not chrome or ie.
The event log can be found in Dnn (Admin > Event viewer)
Just some thoughts,
I hope they help,
Geoff