Somebody is taking over my mobile phone

I have a Nokia 7710 with BSNL postpaid connection.
For the last few months I am having a problem with it, sometimes it stops ringing, or vibrating, in the same time it does not log anything. I have to open the battary and restart it to work.
Now the problem goes more, it is automatically trying to download someting through SIM services(I couldn't find out this feature). It gives an error while downloading, if I press OK it goes out. I also got a SMS from 8543(accidentally I delete that) saying "Please switch off and switch on your mobile phone" !!!!!!
Just saw a incoming call for a second and the phone restarted. No log for that call !!!!!
Can somebody help me to figure out whats going on ??