sim900 questionmarks in serial monitor

I worked really hard on a arduino project with sim900 shield. After some months i finally got everything working. But sinds two days i have a big problem. The serial communication is doing really strange things. I tried shorter wires, different baud rates and different AT commands but everything stays the same and before it worked several months. Also i did try different version of the library. The problem is that the sim900 doesnt react really good to at commends send by the arduino. When watching the serial monitor i get by some at commends a strange error. like this one ⸮⸮⸮⸮T+CMGF=1 while the arduino sended of course AT+CMGF=1 and no reaction of the sim900 takes place. Later in my code i request the signal strengt with an other AT commend and that works fine and get the correct response. Like normally. De response of the arduino with the questionmarks isn’t alsway the same some time more of them are in that line.
I am really sad about how this project ended into an disaster but hopefully you can help me out.

I uploaded my code and a picture of my serial monitor as well

Definitief_totaal.ino (5.86 KB)

I have played with a number of GSM shields, and with all of them I had loads of issues with the serial communication.

The default SIM libraries like to use softwareSerial, which consumes lots of arduino cycles, and should be avoided at all times. For one of my bigger projects, I have bought a I2C to UART chip to address this. Even that cost me oodles of time to make work right.

If the baud rate is too high, SWserial will not work correctly - don't go above 9600. Also, take a hard look at flow control - most SIM libraries configure XON/XOFF - which if you don't match on your RS232 side will also be problematic.

Good luck! - Ronald