Abis fluctuations are mostly reflected by “remote transcoder” call drops, not “internal bss” call drops. But perhaps you’re facing a special problem on your abis?
2 cells of one site… Well, are you sure nobody lock/unlock or reset an equipment in the BTS ?
Just to be sure, you can always check alarms… but IMO you need to ask a BTS expert to check the site onsite (with BTS terminal, I assume ? I don’t know much about equimment)