Hi, I am having similar issues with an RV042G. I don't see any "out of memory" lines in the logs, but I am experiencing the web interface becoming inaccessible (though the router itself continues to work). The only way to regain web interface access is to power cycle the router.
It seems the router was fine until I started inputting usernames and passwords into the VPN Client page. I am still troubleshooting, but if my symptoms continue to correlate with yours, I'll reply back. Otherwise, I'll start my own thread.
Have you gotten any help from Cisco or other resources yet?
Edit: Each time my web interface locked up, it was when I was adjusting the Log settings and hitting Save. So I probably have a different problem than you. Best of luck with yours.
Best,
Tim