K750i reboots itself PLEASE HELP !!!

[lucja]

The phone is unlocked. The only SIM I found that the phone will NOT reboot itself with, is a T-Mobile SIM. But here where I live there is no T-mobile carrier. So I'm just assuming that the phone tries to connect to a T-Mobile network according to SIM and since is not finding it gives up and shuts off the radio transmitter. It is very interesting that at all times I have 5 bars of signal on the screen (but I have no idea of what carrier's signal). On the screen anyways it shows INACTIVE SIM and the phone never reboots. I tried switching off the phone manually from power button and it switches off BUT it won't stay off.. in a matter of seconds reboots (switches back on by itself). Have any of you guys encounter this problem??
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When I put in a Vodafone SIM the phone searches for network, finds Vodafone carrier, connects to it, and after 10 sec reboots. Is the network rebooting my phone for some reason? or rejecting the phone? Or I have a faulty radio transmitter module and is drawing to much current and the phone take the decision to shut off.
In the 10 second window of time before reboot I was able to receive a phone call and hear the couterpart saying "Hello" so I know for sure Radio transmitter is working (but I don't know if it draws to much of current).
Any new ideas?