If you are 4.3 firmware there is a wifi bug which gives the symptoms you are suffering.
There has been a patch rolled out so check to make sure you are on the latest firmware first.
After that try clearing forgetting your wifi connections and then reboot into recovery and clear cache.
You can also turn off Auto Network Switch in advanced wifi settings.
I have no affiliation whatsoever with O2 or any subsidiary companies. Comments posted are entirely of my own opinion.
This is not Customer Service so we are unable to help with account specific issues.Currently using:
Pixel 7a (O2 & Lyca), One Plus 6 (Sfr), iPhone 12 Pro Max (Vodafone)