When posting a question like this you should provide a lot more information i.e.
1. Is this an existing design (product) or you own design.
2. If it is your own design then post the circuit.
3. Also indicate how you are powering the SIM900.
With the above information, helping you would be a lot easier otherwise it is just a guess.
only issuing after this AT+CIFSR "AT" command or at frequent interval or issuing any other "AT" commands. Test using "ATD+" command and test whether it correctly dial and connect with others and don't resetting the Module. If it resetting, it could be power supply problem. are you using LDO with the capability of supplying 2A current and 1000uF/2200uF, 0.1uF, 100uF caps connected at Vdd of GSM Module (if it is your own circuit).
When posting a question like this you should provide a lot more information i.e.
1. Is this an existing design (product) or you own design.
2. If it is your own design then post the circuit.
3. Also indicate how you are powering the SIM900.
With the above information, helping you would be a lot easier otherwise it is just a guess.