Download data rate

First purchase worked for a week or two and I did receive near 400 Mbps, the rate that I am promised by Xfininty service. After that it went to 50 Mbps.I noticed lot of uncorrecteable errors in channels 25,26, 27, 28.  So Netgear sent me a replacement (refurbed unit) Now I get about 280 Mbps. This one does not use the same channels. After 19 it uses 34 and above. Lot of uncorrected errors on Channel 9. Only configurable thing seems to be start frequency/channel for scanning which is set to 0. Anyone else experiencing similar issue. I am in Chicago land.