NanoStation M2 Degradation?

We have installed multiple NSM2 units and are building out a mesh here in Livermore, CA. I've had a link up and running for about two months or more with NSM2 units at either end at a mile apart. We are set up as 2 GHz, Ch -2, 10 MHz BW. When installed and for the first month or so, I was getting over 20 Mbps link data rate in both directions. However, it is slowly degrading to where now it is around 8 Mbps. The weather conditions have been about the same as before. Signals have been around -80 dBm at either end and that's been fairly consistent. Sometimes, the noise floor goes from -95 dBm to -92 dBm, but otherwise nothing is different. I've unpowered both nodes, rebooted both nodes, re-optimized the antenna pointing.... all to no avail. The link seems pretty responsive, although admittedly we don't have a lot on there such as video servers. Is the link speed calculator correct, or is there degradation over time with the NSM2 units (I've seen mention of it)? I really don't think it's link conditions that are changing, but could be wrong!

Thanks,
Dave K3GX