I am starting to believe that my battery issues were 2-3 isolated incidents. Lately the phone is staying a full 13-15 hours without any issues with moderate usage: 3g/wifi browsing, media player + FM transmitter and the rest.
One thing I did is to remove the RSS applet and empty the RSS feeds. I almost never use them anyway. The other one is to shutdown the phone, remove the battery, hold the power button for 10-12 seconds, put the battery back and start the phone. I don’t know if any of those had an effect on the battery life.
I also contacted the Nokia support, since I still believe that there is an issue with my phone battery. They will send me a new battery. I will have a chance to compare the new one and the old one.
I had a chance to chart a “normal” usage and the deep sleep mode for the phone looks like working pretty good. I’m hesitant about posting more charts to my blog, but I’ll do it anyway to be fair to the previous post. Here are the charts.