You should check it in detail, i.e check the utalization in hourly basis at the hour when you have congestion.
Some times it can happen if the cell is concentric then DCS is empty and still we have congestion as call has to meet Level thresholds to qualify for DCS and so non-criterion meeting cells may cause congestion.
It might be that one of the TRXs is faulty or not proporly created so call is never assigned to it.
Similarly depending on which vendor you are working on there might be issue with creation on one / multiple TCH Channels not taking calls.
There might be issue with HR / FR thresholds.
Check stats on hourly basis and on TRX-Channel basis