Official Kies ICS upgrade has left phone unstable - advice???

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http://forum.xda-developers.com – My wife was having problems with her S2 (running stock Gingerbread), so I upgraded it using Kies. The update failed during the download process, so I needed to run the recovery option, but the outcome was the phone was still upgraded to ICS. It's mostly fine, but the phone is now freezing once or twice a day, requiring a cold start to revive it. It also didn't solve the original problem (screen going blank and phone becoming totally unresponsive immediately after dialling a call). The phone is still under warranty, so I really want to leave it in a state which doesn't compromise the warranty (HowTos)