25 ft. for the Signal Strength to drop out indicates that there is a problem in your environment that prevents the signal to propagate (most likely in the building construction - duct work, pipes etc.).
It is possible that the Signal Quality is what makes you think that the "signal drops" and this would be usually caused by interference (noise in the environment).
Normally, to extend the coverage area, you would use a higher gain antenna. However, I'm not sure if there is one for your wireless router (most likely not) or if this would help at all in your case. If the small coverage area is due to building elements that prevent the waves’ propagation, than a higher gain antenna wouldn't help.
Are those 25 ft. of coverage in the horizontal plan or vertical? In other words, you have good coverage in the basement (everywhere on that level), but it drops when you go upstairs? If this is the case, relocating the wireless router on the main floor and running a cable to a switch in the basement, might work for you.
Implementing wireless networks is not a cookie-cutter solution as it's highly dependent of the environment and this is true, as you could see, even for an extremely basic installation.