sorry i did a typo, but you understood what i wanted to do :
— trx pref mark = 2 on TRX1 (to push TCH traffic there)
— trx pref mark < 2 on the other TRXs (and don't forget to put 0 on one or more TRX to allow GPRS)
Then check RMS DL rxLev/rxQual on the TRX1, compared to other TRXs that carry traffic.
If TRX2, 3 & 4 show equally bad DL RxQual, it is a Freq problem.
If TRX1, 2, 3, 4 show bad DL RxQual, then it means your BCCH is also interferred.
If only TRX3 shows problem, then I would strongly suspect a TRX problem.
regards
pix