This requires deeper troubleshooting. It's not necessarily related to the upgrade.
Try to correlate if the problems happen always in the same areas.
When facing the problem, can another phone work fine next to a phone having an issue ?
When facing the problem, can a laptop detecty nearby APs with good signal strength ?
When the issue happens, does it solve itself and if yes how quickly ?
You can try a debug client on the WLC and check the point where the phone starts to face the problem. It will tell you if the WLC kicked the phone away for some reason or if really the phone has trouble in finding an AP (RF issue ?).
Were you able to load an ER file once you upgraded to 7.0.98? I read you were supposed to use 220.127.116.11 ER but from my knowledge the 5508 WLC doesn't support 5.x code so I am just wondering how or what ER file you are supposed to use because on my 5508 WLCs it says N/A. So, I am just wondering if your WISMs say the same thing.