Cisco E3000 cuts my internet speed (usually)

I pay for higher speed DSL (don't laugh but here in the boonies that is 3Mbps instead of 1.5Mbps). However if I put my Cisco E3000 in between my modem and my computer (still all high-speed LAN cables, not using the wireless) then my speed drops back to around 1.5 (most of the time, like 6 days out of 7. I get amazed on that one day because I'm like "see it isn't at my end").

Now I have it locked down and my other computer that uses it wireless is not even turned on so I know it's not the neighbors stealing my bandwidth. And every time my ISP checks they say everything is working normally but they also want me to direct-wire the computer every time.

So any ideas what would cause this? I'm not an expert at routers but I bought what was supposed to be a pretty good one, dual-band etc but I can't accept this loss of speed. If I have to I will only connect the router when I need to use my HTPC but that kinda sucks.

I'm not laughing. I am out in the country (boonies?). I am in one of the few remaining villages that still has copper wires, although the promise is optics withinh the next five months. My router (i have three different models) does not control my speed. The most our, ar any, ISP can promise us on the shared copper is 4Mbps, to which I contribute.
The catch here, maybe relevant to you, is that the contract states clearly "up to 4Mbps. The ulimate speed is dependant on how many of the people in my lane happen to be sharing the copper cable at the same time. I have never, over many years, had more than 2,4. I've got to live with it now.

boonies = boondocks.

And yes, it is up to 3 and they promise 80% so when I get 2.4 they say it's within limits and I say OK I will mail 80% of the bill but for some reason THAT isn't OK.

But what I'm looking for is ideas why the router could cause such a loss in bandwidth. Some setting I'm missing maybe. It's really all pretty automatic, all I did was lock it down with a password and disable ID transmission.

In an attempt to improve mine, I googled quite a bit. Found things like changing the channel, putting on a high gain aerial etc. Nothing helped.

That would be to improve the wireless signal, like to my HTPC in the living room but here I'm direct wired by LAN cable into the back of the router.