Update -
I spent a long time on the phone last Sunday with Dell support. They went into my PC and could not find anything wrong with the Dell 1506 wireless adapter software settings. I asked for a replacement wireless adapter of a different model, which they agreed to provide. They shipped me a new adapter which is an Intel 2230.
I removed the Dell 1506 adapter and installed the Intel 2230 and it has made a huge difference. My wifi signal is now reading 5 bars pretty nearly all the time and I've had zero problems with the internet connection. Using the new adapter, inSSIDer reports a wifi network signal strength of about -50 dBm, whereas with the Dell 1506 adapter the signal level was usually around -70 or -80 dBm, so I have gained 20-30 dBm of signal: That is an amazing factor of 100x or 1000x improvement in signal power.
To me that says the Dell 1506 was defective hardware. The replacement with an Intel 2230 seems to have solved the problem.
Thanks,
-S