how much is “a lot” ?
could you check some QoS Stats :
– idle band interference measurements per TRX
– RMS path balance per TRX
then check your frequency parameters (hopping list, maio)
it could be due to a sudden loss of coverage (elevator, stairs, deep-indoor meeting room). To isolate this case, you can activate a microcell cause “bad SACCH”, or you can increase A QUAL HO and reduce A LEV HO.
The best would be to “walktest” ths building…