I can confirm that and my provider is Vodafone PT and I never had such problems before so it's not my sim card.
So I ordered a new SIM card (same number, 128KB SIM). I put the card into the phone and after two hours of running 3G without any problems (R1). So I upgraded to R2. Now it is 29 hours, the phone is switched on. During that time, the phone never once lost signal nor does not restart. Use as a business phone, so SMS and frequent calls are commonplace. Automatic switching between 3G and 2G started to work without problems. So that really was the solution signal dropouts and restarts (due to loss of signal) replacement SIM card?