Charging phone-sometimes hot sometimes not iPhone 4

When the iphone reaches a full charge, the load on the charger decreases drastically. Maybe its those times that the iphone was fully charged a while and the charger cooled down.
Just a guess....
Dell-WinXP, iPad 3G, iPhone 4

I also noticed that my wall charger was getting pretty hot when charging my iPhone 4. I didnt notice this with my two year old iPhone 3G. I was surprised when I grabbed the adaptor after just finishing up the first charge on my 4, to notice how warm it was to the touch. Not enough to burn, but quite warm - like someone running a high fever.
It happened the two times I used the original charger and I called AppleCare about it. I was worried about a potential fire hazard. They said to return it to the Apple Store and get a replacement. After a bit of checking, the Apple tech gave me another adaptor. The replacement still gets warm, though not quite as hot as the original one.
On the other hand, my wife, also with a brand new iPhone 4, says her adaptor stays cool. Is my iPhone drawing more current than hers when charging? Or, am I unlucky enough to get two defective adaptors? Whats going on?
iMac Core 2 Duo Mac OS X (10.6.4) iPhone 4 32 G

Whenever I charge my phone from say 20%, as it charges up, the charger gets warmer and warmer. For every 10% charging, theres twice an increase in temperature. So when it reaches about 50%( 3 times) the charger is pretty hot and the USB part that goes in the charger is BURNING hot!
Ive noticed this doesnt happen when I charge from 90% to 100%
So I charge in intervals and its very time consuming. I have to keep unplugging the charger and letting it cool for about 5 min and charging again. This is very tedious.
Once I left my iPhone on charge(9%) and forgot to do this practice, I came after an hour or so to find my iPhone stuck on 83%. The charger had burnt and wouldnt function.
I live in a city with no official Apple store so I couldnt get it replaced.
The new charger I have has the same problem.
I love Apple, a lot. Someone help please.
iPhone 4 iOS 4 Black iPhone 4 16GB GSM used both with 110V and 220V