Here are some things to check when trying to handle wifi issues.  These are not in any particular order.

• Update your NIC drivers and your WAP firmware.
• On your WAP, check and see if “bursting” or “framebursting” is available and on.  If available, it should probably be on; but check for multiple framebursting modes, there are proprietary modes.  For just one example, on some Buffalo WAPs, there are two such modes, “125” and “FrameBursting”, and although “FrameBursting” is compatible with many other vendors’ wifi equipment, “125” can cause problems.
• Ditto #2, on the client side.  Intel WNICs often have it (called “bursting”) turned off.  It should be turned on, but watch for special proprietary modes, stay away from these.
• Sooner or later you will need to do an environmental survey.  This is the slam-dunk cause in many a congested situation; if you have ten neighbors with wifi, you’re likely to have four or five trying to use the same channel. One of the better ways to do this is to get a DD-WRT -compatible WAP, put DD-WRT on it, turn DHCP off, put it on a unique private subnet and IP (say, 10.9.8.7), plug it into the LAN, and use static IPs on a laptop or desktop to use its utilities, which are in the Status area, under Wireless.