Have you tried Googling radio core catfailed? There are quite a few articles about it. It looks like keeping the phone on 2G is the most reliable way of avoiding the error. Some people seem to think a firmware update can help. Try the latest official update from your telco. If there isnt one* avoid companies that dont let you switch between 2G and 3G (e.g. AT&T in the past)
Sorry I cant be any more help* but 5 minutes on Google should give you quite a few suggestions.