I have used varnish cache for awhile on my Magento 1.8. It worked well for a long time until now I'm having an issue that some product URL got 404 error. It's randomly, not specify any product. And the product worked again after I flushed the varnish cache. Note that I haven't changed the VCL since the day the site started and The extension Nexcessnet_Turpentine was updated to 0.7.3. I updated all the magento security patch. What should I do to debug/solve this issue?