Could be the RDS function that is causing radio to change channels.
Look at your handbook and it says that RDS will cause the radio to tune into a stronger frequency that carries the same programs (I think this means it chooses another station with same content, eg, pop music, news etc to the one you are listening to). It also tells you how to turn RDS off, because its one of those things that comes on automatically when you switch radio on. -To switch RDS off, press PWR/VOL knob to turn radio on, press PWR/VOL knob for more than 2 seconds, and press the PRESET 1 button, then press PWR/VOL knob.
The 'TP' indicator is not under your control and will display only when you are tuned into a station that broadcasts traffic information, which you can recieve if you press 'TA' button (top RH button of radio)