The problem happens only on some cells, so it’s probably a radio problem. I would suspect that the radio conditions are not good on this BCCH, and perhaps you’re requiring an init coding scheme which is too “high”.
Try reducing the init CS in Dl and UL to CS-1.
By the way, what is “ASR” ? Is it estab success rate or “retainability” success rate ?
Do you mean there is a TBF drop=70% for GPRS transfers?
But how many transfers in GPRS are there? Many phones are EGPRS capable. You are perhaps worrying about RA update of very old phones which are not able to perform well in GPRS ?