Lots of chargers work to top off an iPhone, but few work to keep an iPhone charged up when under a heavy load.
I too have experienced an iPhone's charge going down, down, down EVEN when plugged into a car charger (500 mA or 1/2 an amp). Half an amp is enough to charge up an iPhone IF you aren't using GPS, Bluetooth, Wifi, 3G, iPod or a combination of several. BUT, if you are using these and/or your iPhone falls below 20% charge, it CAN NOT charge your phone up or keep it charged.
As several posts have indicated, you need 1 amp (1000 mA) to keep up with this heavy use. This is the charge that the Apple-supplied home charger provides.
After several losers, I FINALLY found an excellent solution that can keep my iPhone powered when running GPS on Google maps, actively searching for wifi, bluetooth on AND playing the iPod and in a 3G area (sometimes I was in Edge).
It can support a FULL TWO WATTS. The iPhone can only draw 1 amp, so no worries there. It is made for GPS systems, but comes with a standard USB port so works beautifully with the iPhone. I found out about it because somewhere I read that Scosche built one that worked. I couldn't find it on their site, so I called their tech support. A guy there told me what I typed above about the 500 mA stuff. He said the GPSCHRG model would work for me. I found it in Walmart with their GPS stuff. I used it on a 8-hour trip over the holidays and it rocked! It was about 25 clams.
check it here:
http://www.scosche.com/products/sfID1/210/sfID2/337/productID/1641
