Self Interference Question

I was wondering if it’s possible to cause self interference even if your tower sites have sync? I have many subscribers that can see multiple AP’s. Some of these AP’s are even on the same channel. My thought process is that even if these AP’s are timed, the subscriber is seeing the output from the AP’s at the same time.

Has anyone had any problems with actually having too many tower sites in too close of a proximity causing problems? What are your thoughts on possible workarounds?

I was having some issues earlier on with our 900MHz access points, but balancing the RF power and using color codes eliminated the problem and definitely improved the overall performance of the affected APs and SMs.