Hi paul,
A piece of very important advice : always activate power control (dl and ul), especially in Urban Areas.
The problem is that your parameters are wrong. Try this :
L_RXLEV_DL_P =-80 ***
U_RXLEV_DL_P =-70 ***
L_RXQUAL_DL_P = 30 x0.1
U_RXQUAL_DL_P = 10 x0.1 ***
A LEV PC = 4 SACCH
A QUAL PC = 4 SACCH
The rxlev will decrease, as you are experiencing, but it will remain acceptable. It will strongly help to decrease your network interference.
regards
pix