Well I posted a seperate thread in Android Lounge about rooting with a view to perhaps solving this problem. Here is the link:
Could rooting be the answer to custom contact ringtones problem?
The reply suggested it wouldn't perhaps solve the issue, but questioned whether the problem might be my SD card, in particular the class (therefore speed) of it.
I 'think' the idea was that a higher class of SD card was needed to ensure faster access and transfer of the ringtone so as the phone can access it quickly on demand (i.e. when a caller with a custom ringtone assigned calls the phone).
If I have understood it correctly, that would seem to make sense to me - not that I'm well clued up about these things, of course!
I have an old SD card taken from an old Samsung phone of mine (clearly from before they even began classifying them!) - just a Samsung brand 2GB unclassed one, so I'm wondering if the poster - lunatic59 - may be on to something.
So out of interest, what 'class' of SD card are you using socrates0? Do you agree this might be the problem - before I rush out to buy a replacement SD card! As obviously, if you are using a faster class of SD card and have this problem anyway, it might not be the answer after all.