Signal strength is not a good measure. There are situations where the MR33 may reduce its transmit power - because it doesn't help the throughput. Perhaps try using something agnostic like www.speedtest.net (assuming you have a fast enough Internet circuit to use up a lot of WiFi bandwidth).
If you want lots of coverage then the 2.4Ghz band is best, but the trade off is reduced throughout.
If you want lots of throughput then the 5Ghz band is best, but the trade off is reduced coverage.
Personally, I try to disable 2.4ghz and only use the 5ghz band.