This is not a VPN issue. This is a network using WPA encryption issue. WPA processes the key during an interval and passes its data through a MIC check which is an integrety check using the temporary key responsible for rekeying. If you look at your router and see the rekey interval, I assume they are dropping right when that interval hits?
I suggest you reconfigure the key and reboot the AP and see if that fixes anything if this is a widespread issue.
Thanks buddy, I just changed my preshared key and click on "activate changes" in the web gui interface. Or give this command "exec wlan reactivate" so the changes are re activated. and now it work like butter... thanks a lot