I had this issue me too after OTA to HTK75_WW_epad--220.127.116.11.
When screen was off wifi was endless trying to reconnect.
Closing the pad was fixing until new screen off.
My router is an old WRT54G V24-sp2 mini
I found this fix:
Network mode: G-only (no n-mode with Wrt54g)
ACK Timing 0 (Default value was 2000.)
In this way no need to have DHCP time down.
I really dont know what ACK timing is used for, but this give me a fix!
Hope this help.