There should be a logging feature which you can temporarily enable on the router to get more insight into what's going on or wrong.
Do you have to reconfigure the router from scratch after the firmware upgrade?
If you find that an earlier firmware version works better for you (although normally later should be better) you can always downgrade to the earlier version.
Have had bad experience with Linksys kit previously. Had a dodgy bridge that only worked for 6 months. They agreed to replace it but insisted that I return the old one to the nearest UPS hub - which was 2 hours drive away - so not worth the journey.
Only plus point with them is that their online support works well but I've steered clear of their kit since..