Internet Speeds and Low Downstream Power

For the past week or so, I have been seeing severely reduced internet speeds on my connection.  I have the 500 Mbps down connection but at most am only getting ~300 Mbps.  I have a Netgear CM600 hooked up directly to the cable drop, no splitters between them.  Before I was able to get 500 Mbps down consistently.  Nothing has changed with my setup, so I don't believe it's anything with my setup slowing it down.  My modem is showing 9 locked channels, with downstream power levels ~1 dBmv (one at ~4 dBmv) , and all SNRs are about 40 bB.  I seem to recall my downstream power levels being higher.   I have reset my modem both through Cox's service and locally (hard power down and software reset).  Neither significantly changes the downstream channel data.  Could the low downstream power be contributing to the lower connection speeds?  Or is there something else I should be looking at?