iPhone Not Charging While On A Call

My iPhone 3G (currently running 3.0) refuses to properly charge while I am on a call. I used to be able to talk and charge at the same time, but lately, it shows the 'charging' symbol for a while and then just switches to 'connected'. The first time it did this, I realized too late and was left with a long and boring commute to work.

I am quite sure it has nothing to do with the update to 3.0, because it started doing this a day or two before I made the upgrade.

I usually connect to my laptop when charging, but I also tried connecting via the wall charger, which resulted in the same problem. I don't think it's a problem with the cord I use; I tried it with my Dad's too and it does the same thing.

The moment I hang up, though, I am able to charge my phone with no interruptions. It continuously shows the charging symbol without randomly switching to just connected.

I have a habit of making long, long phone calls and I like to have a fully charged phone when I leave for work or whatever. Does anyone know what the problem is/might/could be and/or how I could fix it?

Thanks.

Note: I was unsure under which topic this issue would be most appropriate and decided this was the best one. Please don't flame me if I'm wrong.