In case you find the problem on all sectors + all trxs then will it be possible to provide your 1800 with another transparent E1. Believe me but the same problem in our network was resolved after providing a transparent E1. The BTS and BSCs were of Huawei.
Also how could be FER and BER poor with good SQI and RxQual. (Are you looking at FER Sub or FER Full) ?