Mindstorms EV3 unable to drive two motors simultaneously
Most people with port problems have port C failure. I had similar issue. In my case it was factory defect: SMD chip (U26 on board, see image and HW dev kit pdfs) was soldered with a twist and had two legs broken, but they touched the chip and sometimes signal was getting through. They are used for tacho input. So in some modes when it runs fullspeed it works OK (remote control or built-in motor control), but when it is required to turn for degrees or rotate with a given speed it just spins fullspeed without a stop since it does not getting tacho from the connected motor.

I replaced the chip (74HC2G14GW-G) and now everything works fine. I don't think they solder chips by hands, so it could be some defective batch.