I have a 2926 dual wan with DSL and CABLE connected.

I seem to have unreliable access to internet. Many pages "cant find server" then they will work later.

Same on LAN or WiFi

I have most recent firmware

Status shows both WAN green

Are the connections with different isps?

I would be looking at your load balancing settings.

I have 2 WAN connections but route each device (or groups of devices) to one connection as first priority.

In Load Balance/Route Policy I found "advance mode" worked best.

I see the same thing very occasionally.  I'm pretty sure I didn't see it at all until fairly recently, maybe a few months ago.  This would coincide with me changing the DNS server on my router, PCs and mobiles from the the default (my ISP's DNS) to Quad9.  Or it could be related to a recent router firmware update.  For me it happens so rarely that I don't plan to spend any time investigating, but if you recently changed your DNS too then that might be a clue.

My router is a 2866Vac on DSL.