for interference : are you certain the UMTS bandwidth is not overlapping with your GSM carriers ?
Are all GSM frequencies interfered ? (i don’t think so)
How are you connecting the BTS and the antennas ? Are you duplexing/combining 2G and 3G onto the same feeders ?
regarding the 3G/2G HO, you need to analyze the problem:
– is the HO detected ? (see the 3G qos stats)
– does the RNC send the HO REQ to the BSC ?
– does the BSC receives it ?
you have to know at what stage the failure is occuring.