Sudden and severe loss in AirPort reception

Question

I use my TiBook at my desk (12 feet from wireless router) and on my couch (9 feet from wireless router). AirPort reception has been very strong (always five bars) for more than two years in these locations. I was even getting good, consistent reception throughout my entire condo. I have moved/added no furniture nor any other possible sources of interference.
Suddenly, reception is severely fluctuating and impacting network communications. I am never getting five bars, now. Reception varies wildly and quickly: 15 seconds of four bars, followed by 10 seconds of no network connection, followed by 20 seconds of two bars, etc. This continues even if nothing changes—for example, if I step away from the TiBook and watch the screen from several feet away.
I have tried the following without results: reseating the AirPort card and confirming the antenna is securely attached, pressing against the inside of the battery compartment to reseat one antenna, rebooting the machine (several times), small adjustments to the router antennae, subtle repositioning of the router, reseting the router, setting wireless communications from G to the supposedly more stable B, trying several channels other than the default channel 10. Nothing helps.
I have been reading about connectivity issues, but nothing suggests I should be having AirPort troubles when only 9 feet away from the router! I have run out of ideas. What am I missing?
J.

Answer 1

And another afterthought: I just picked up on rereading your post that you live in a condo. That suggests the possibility that someone in an adjacent or nearby unit may have installed something new in their unit that has caused increased interference for you. You obviously have no control over that — perhaps not even the ability to discover whether it's true. If you can't find any indication that either your router or your Airport card is malfunctioning, you may need to switch to using a wired connection, inconvenient as it may seem.
In my own home (which is several hundred feet from any external interference source), I was stymied for a long time by limited wireless range to distant parts of the house, porch, deck, etc., and I couldn't face having to run ethernet cables all through my existing walls. I solved the problem with two of these ($48 for the pair, shipping included, on Ebay). They work extremely well in my situation, but they might be vulnerable to power line noise in yours; I don't know for sure. Of course if you only use your Powerbook in the same room with your router, using a simple patch cable to connect them wouldn't be much of an inconvenience compared to having your connection dropped unpredictably.