The problem happens on one of your Pis? no matter which module is connected?
The power us pulled from 5v... there is a LDO which converts this to 3.3v for the GPS module annd 1.8V for GSM module.
It could be a power issue with one of your Pis
Try connecting it via USB only. You will have to access the NMEA sentences through the USB port. Instructions here:
If you still see the same problem, try increasing the current to the USB ports.
add the below line to /boot/config.txt
Mark --OzzMaker.com --