Linksys routers, extremely slow download speed

I have a pretty basic setup at home.  Cable modem into wireless router, single PC out from that, and a few wireless devices.  I have Mediacom Cable internet service, I pay for 18Mbps down I think, but I usually get 27-28 on speedtest (don't tell them).  

I've had a Linksys WRT350N wireless router for about 5 years and never had a problem with it that I can remember.  I never had to restart it to fix speed issues or any of those kinds of problems.  A week or two ago it suddenly (from one day to the next) had severe download speed issues.  I hadn't done anything that I can remember the night before, I think I watched netflix some, but didn't install or update anything.  Speedtest would show my download speed start by spiking to 0.5Mbps or so and drop off to nothing, then halfway through spike again to the same and drop off.  Upload speed seemed to be unaffected, getting 2-3 Mbps which is about what I was getting before I think.  Speed problem affects all devices that go through the router, both wired and wireless.

Plugging PC straight into modem resulted in immediate jump back up to 27-28Mbps.  No spikes, speedtest was a solid plateau of speed.  So I restarted the router a few times, reset to defaults, updated firmware, flashed dd-wrt, tried all the settings I could find mentioned on different forums.  Tried disabling WMM support on QoS (seems to be the usual fix for this).  Nothing helped.  Luckily I had our old WRT54G still in a box, so I tested that and it worked perfectly.  Full speed, no problems at all.  This is with all the same patch cable, ports, devices, etc as I was using on the 350N.

So I figure my router died of old age, oh well I got some good years out of it.  Bought a Linksys E1200 N300 refurb on sale and it arrived a day or two ago.  Hooked that guy up and same problem again.  This one is a tad different in that it will spike a bit higher than the first on speedtest.  Once I saw it start at 0.1Mbps and slowly ramp up faster and faster to where it hit 9 Mbps by the end of the speedtest.  But most often it just times out trying to acquire a ping and server, or sits at 0.1 Mbps until I close the window.  The last time I tried it speedtest wouldn't even load.  Checked all the same settings as before (QoS, etc), reset to factory defaults again, same stuff.  Plugged the WRT54G back in and get full speed immediately.

The fact that the 54G works fine tells me the patch cables are probably fine, as well as the PC that has been hooked up in all test cases.  I tried spoofing the MAC from the 54G onto the E1200 in case it was something crazy with my ISP caching.  I tried flipping lots of settings around on the E1200 to see if they helped, turned off ipv6, turned off wireless, turned off firewall, checked logs (empty).  I haven't tried dd-wrt on the E1200 yet since I might need to RMA it.

There are a lot of forum hits with similar problems, but most of them are fixed by disabling WMM.  What have I missed or not ruled out?  Open to anything.  The only answer I can think of that fits the symptoms is that my 350N died of old age and I got a bum refurb for a replacement.  Seems unlikely since the problem is so similar though.

Appreciate any advice.  Except maybe the typical "E1200 is a crappy router" comments.  I know it is, but it should still work.
ANPACAsked:
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PerarduaadastraCommented:
It does rather sound as if you've got a duff unit.

I'd be inclined to buy a new wireless router; they're not that expensive, and you'd have some kind of support and a warranty to boot.

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Matt VCommented:
I had the same problem with the 160N... worked fine for years then suddenly it was no good.  I ended up buying a Cisco 871 off e-bay and it works flawlessly.
ANPACAuthor Commented:
Thanks.  Made me feel better hearing someone else say it.  RMA'd it and ordered an average Medialink router.  Immediately saw speed return to normal.  I may be done with Linksys for awhile.
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