Are you running the test within line of sight to the router? Can you also run a ping test (not pingtest.net!) to the router 500 times?
If your ping is spiking to more than 10 ms, your speed goes down due to high latency. An excellent ping test to router result should give a summary similar to this one:
Ping statistics for 192.168.1.1:
Packets: Sent = 500, Received = 500, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 0ms, Maximum = 3ms, Average = 0ms
It's not just you who have this kind of problem if you read some threads here. I used to have those problems before but after using Dell's 16.8.0.6 drivers and Windows 7 updates eliminated this problem for me. I found the "problem free" driver that works best and gives all the speed (150 - 180 Mbps actual throughput of 300 Mbps link speed) for my laptop (Sony Vaio with Intel Dual Band Wireless-N 7260) and won't ever try updating to whatever latest Intel drivers. I've seen that many people get excellent speed with 16.6.0.8 though that driver gives erratic connections to my laptop