Cannot stay on a call more than 20 minutes because phone goes into Airplane mode?

[sa330206]

I have had the Touch Pro 2 for over a year and constantly have the problem of it going into Airplane mode when I'm on a call. After about 20 minutes the phone goes into Airplane Mode turning off the phone and drops the call. I had the problem with my first TP2 but I dropped it in water by accident and got a new one using the replacement plan. The new one does the same thing. I mailed it into HTC and they confirmed a problem with the circuit board and replaced it. This still didn't fix the problem so they sent me a new battery. I am now in the process of sending in my phone for the 3rd time. I've looked around and can't find much of anything for this issue. It appears the battery is getting hot when the phone goes into airplane mode. If you call back the phone will start going into airplane mode every 5 minutes or so. Anyone heard of this or have any suggestions?? I've even offered to take a lesser phone but HTC insists on me to keep sending it in but this has become a huge pain. Thanks in advance!