Uninstall all device drivers and all ground control stations, reboot and re-install ONLY the latest mission planner, (it will automatically install the correct device drivers). If that does not work check your cables and hardware.
I guess you have already checked the physical connection between the radio and autopilot (maybe your link has some lines swapped or bad contact). Normally there is also a red light on the radios which blinks each time a message is send and normally you should see this also as the autopilot sends heartbeat messages (~1 per second). Once connected it off course flashes much faster.
Maybe also double check the parameters (while connecting directly with a USB cable). Check that SERIAL1_BAUD matches your radio baud, typically 57600.