First disconnect everything. Find your NID (network interface device, aka DMARC) and plug the modem into the customer side (you likely won't have access to the ILEC side) and then connect a laptop to it and set it to ping with a 200 ms interval to your ISPs primary DNS server overnight (unless it is in a location where this is not possible, then you will just have to sit with it for a few hours to see if there is any visible loss on the circuit). This test will let you know if your ISP is the problem or if it is on your end. If you find it is the ISP, call them up, tell them what you did and send them the ping results and they will do the rest.
If you find that there are no issues there, connect it back up where it was before, with only your laptop again, and perform the same ping test. This will determine if your IW is to blame. If this test fails, then you need to replace the cabling between the modem and you NID.
If you find there is no issue there, add you router back into the equation and run the ping test again. You should notice a pattern here. Basically start with the fewest variables and rebuild your network until the issue presents itself. At that point you know what is causing the issue. If you do find it is on your network, it may be prudent to try a TCPDUMP to watch the traffic and see what is happening to it when you experience drops. You can often narrow it down to a specific interface that way.