Red X is supposed to mean "hardware problem". The only way I can think of that it would be intermittent would be intermittent bad data. Bad power might cause that. Also, changing the GPS autoconfigure for ubx/OP GPS's can cause it. Some kinds of the clone DJI GPSs send a lot of bad packets (like about 1 per second), but work OK with the packets that get through. Are you using a switching BEC (efficient but noisy) or linear BEC (inefficient, hot, but clean power). Also, if you have any kind of transmitter in the aircraft, you might switch it off for a test. Even modern RC receivers transmit data back to the transmitter. Just things to test.