the DL interference can be found easily by checking your frequency planning on a map.
the idle band measurements are only about the UL interference !
since you already changed the frequencies many times, I suggest the problem is not interference 🙂 when changing frequencies, do you see any change in the ratio of HO DL QUAL ?
Don’t you think it could be a Coverage issue ? Bad RXLEV in an urban area is usually linked to a bad RXQUAL. The HO QUAL might be triggered before the HO LEV. What is the ratio of HO LEV (UL & DL) ? What is the ratio of PBGT HO ?
What is your TCH allocation efficiency per TRX ?
What are the HO Success Rate per neighbour ? (type 180, per adjacency, not per cell)
What is the TCH Erlang per TRX ? Try increase the TRX PREF MARK (from 0 to 1 for example) on the TRX that carry the smallest amount of Erlangs, and oppositely, decrease the TRX PREF MARK of the TRX that carries the highest amount of TCH Erlang.
Regards
pix