On a Dell Vostro 3550 we have the same issue.
I was playing around with several settings of the driver, but all did not help.
A ping directly to the access point will result in periodic 7-8 pings, 2 timeouts 2-3 very slow pings, while a reference system with another intel card (3945) works without any flaw.
There is one curiosity: If you start a wireless scan in the background - e.g. Metageek´s InSSIDer, the connection will stabilze, but response times still vary between 20ms and 200ms, while the reference system pings at almost constant 5ms (at most).
An external (USB)-WLAN-adapter turns out to be uninflucenced by this issue.
It seems to me, that some driver or bus-throughput or energy saving options fight against each other - which would also explain the periodic recurrence of the timeouts.
From my point of view, it seems to be an intel driver bug.
I found following description:
This article describes compatibility problems between routers and energy saving functions in intel ProSet Wireless adapters.
The "alternative solution" is to put the adapter into CAM Continuously Awake Mode.
In Windows 7 64 bit with latest intel drivers installed, this description seems to be inaccurate, since the described steps do not apply, because the elements are not there. Instead, you can enter "Windows/Control Panel/System/Energy options /change energy savings plan/Advanced" now select "Wireless adapter settings/Energy saving mode" and select "maximum performance" for both battery mode and ac power and quit the dialogue with "Ok".
(Steps translated from german Win Pro 7 to english, they may actually be named slightly different).
After applying these settings the connection is stable.
In case you have activated "Auto" für channel bandwidth, this may cause other problems, as in my case, disconnects. Reverting these settings to 20 mHz seemed to solve this problem too.
Affected Router: Cisco/Linksys WRT160N v2. fw 2.0.01 (RT2880F chipset)