Gsm800c uncertainty

I’m facing a problem of uncertainty of sim800c when used with stm32f103c (bluepill) module. I’m trying to send sms to the module continuously where I only get some of the messages clearly, others contain junk like characters in them. I’ve attached a screenshot of the com port containing the sms received by the module.

P.S: I’ve marked the correct content with green color and incorrect content with red color.

Have you used proper level translation between GSM module and the the STM32 UART lines?
The above problem can occur if the power supply is not clean and UART lines are carrying noise on them. The clock speed of STM32 also makes this occur sometimes, Make sure the clock selected is the right one. It should be fast enough to catch the data on UART at set baud rate.