Cause of Drops is not clear.Drops come randomly and majoraly are pegged in TCH_RF_Fail
In afternoon with almost same traffic we do not observed any drops.
But in evening i.e. in peak hour lot of drops are observed. And Most of the BSCs having such problem belong to same MSS.
Earlier their seemed to problem in SYNC.Every time we changed the clock source,we got improvement in drops. But now we have dedicated SSU for clock generation.So there is no problem of clock as well.
by common cells I mean that suppose there are 100 cells having TCH Drop>2% on day1, then on day2 there are completely different cells having high Drop.Hardly 10-15 cells are common
One of the facts i agree with you is that, yes there are lot of handovers in peak hour and more than 50% of them are due to poor down link quality.