Survey the possibilities
If you can't find a clear channel and still have problems after locking your clients down so that they don't go straying to other WLANs, it's time to see if you can do something about all the unwanted signals bombarding your poor little WLAN, i.e. reduce the RF noise. This is an area where understanding the problem is especially important before implementing a solution and that means you'll need something to measure signal strength to help you perform a site survey.
A "site survey" is just a fancy term for walking around with something that can measure wireless signal strength and recording what you see. You can do this with your wireless laptop, provided that its client application has the ability to show all in-range APs, their channels and some indication of signal strength. It doesn't matter whether the signal indicator reads in %, dBm, or no units at all, or whether it measures signal strength, signal quality or both, since you'll be looking mainly for changes in whatever indicator you use. It's also helpful if the client utility has a fast-responding, real-time signal indicator for the AP that it's connected to, and even better if it can plot the signal strength over time.
If your client utility doesn't have any of these features, you have a couple of choices, both of which will probably involve purchasing a new client card. This isn't as bad as it sounds since the price of 11b cards has dropped down to around $50. NetStumbler is a great free tool for checking out your wireless neighborhood. It includes signal and noise plotting capability and comes in versions that run on Windows and PocketPCs. It works with cards using the Lucent (now Agere Systems) Hermes chipset, which includes the ORiNOCO 802.11b card, but check the release notes for the version you download for a list of other supported cards.
If you'd rather go with a commercial solution, the client utility that comes with ASUS WL-100 and WL-100g cards is excellent.
Once you have your signal measurement tool, take it to your problem location(s) and see what it can see. Since you've already addressed the SSID-related problems (right?), you're mainly looking for the signal levels and channels of neighboring APs. The APs most likely to be causing you grief are those on the same channel and with signal levels greater than or equal to that of your own AP.
Once you understand the wireless environment your client is operating in, you're ready to take steps to change it. Most of the techniques at your disposal are described in the Wireless LAN Performance Improvement NeedToKnow, but my general advice is to focus on solutions that reduce interference from neighboring WLANs vs. boosting your own signal and creating a problem for someone else. Many times a little aluminum screening, intelligent use of directional antennas and just relocating your AP can go a long way.