It could be your wifi router too. I went through 3 (modem + router/wifi which had the worst and I don't like having both on one unit) and cheap $40 router N300. Currently getting 24Mb on wired and wifi can vary day to day but do see 20Mb on a good day and in same room.
Wifi extenders are iffy.
Some posts seem to point to the Mac Pro case as possibly shielding and reducing reception which sounded logical if true. Lots of issues with the 2011 model MacBook Air and others, also metal I assume.
We had a storm, first real surge I have seen and knocked 50% modem and utiilty port drop so they finally came out after two weeks and replaced both. I now have a 'backup' modem of my own just in case (SB6121 seems to work where the cable co's struggled and unreliable, so it seems). The Motorolla seems to bond more channels better though.
I would not go with Airport as being any better except for those that like the idea.
And I would try find a way to hide but use wired - I went down into basement and back up! with small 1/4" drill. We have 12/1 or 20/1 and then packages for 30 and 50. One local FOIS says they offer 100Mb but not sure but out of my price range for sure.
My Netgear can go months w/o needing a reboot or any intervention, very satisfied. You might want to take a look at their USB Wifi and also Extender.
Is there: any interference? wireless phones or other device in proximity?
The most likely problem once you have reset the Router and re-entered the configuration in your Mac from scratch is Interference. To get a handle on that, download a stumbler program like iStumbler and use it to see what other stuff is nearby. You may be shocked to see how many other Routers are around you, all competing for the same channels.
Thanks for the replies guys
I think I'll just go wired. In which case, the question is... do I run a 30m ethernet cable across my flat? Or, do I run a more discreet modem cable across and just move the modem and the router next to the computer?
I'm prepared to do which everone gives me a faster speed
You can move the modem OR Router OR computer to give you the best of all worlds.
Ethernet cables should have all four pairs present, and are limited to 100 meters EACH.
For ADSL over copper wire, the limit is several miles from the telephone Central Office in your town.
For Cable-TV wiring, there is no practical distance limit, but you need a good strong signal when you are done.
If you do decide to add wireless devices later, there tends to be a small dead spot right at the antenna.
So the technical answer is, whatever appeals to you.