Strange latency problem

Some friends of mine have a D-link "N" router (with two aerials) supplying wi-fi around their house from a Virgin cable modem. When wired to the router a laptop will show around 15 Mb/s, pretty damn fast!

Most of the walls in their house are solid brick so as they move away from the router into different rooms internet performance decays considerably.

However...
the apparent signal at the laptop always looks reasonably good, and actual download speed is OK too (measured using Speedtest.net). However the problem is after clicking a link it can take 15 - 30 seconds to respond at all and then it will "stutter" with short bursts of quick downloads followed by annoying pauses. Video is usually impossible.

Normally when signal decays as you move away from the router it is the speed that drops not the latency. Actually I am surprised they get a signal at all in their kitchen which is shielded from the router by three solid brick walls!

Is this stuttering performance normal behaviour?
Is their anything they can do about it?

They would happily sacrifice some raw download speed for a reduction in the pauses and stuttering!

often I see 802.11g and later standards flapping around between speeds with a pause to change gear - keep an eye on the radio connection speed it may be all over the place, if it is then set the router (if poss) to a lower speed or slower standard.

Also see if it's the same with an IP address rather than a URL - in case it's a DNS problem.