1995 s14 200sx ABS HELP

Guys, can anyone help me with this problem that I'm having with my S14. I have seen this same problem on another S14. The problem is sometimes when the brake is applied the pedel becomes very hard, and there is absolutely no brakes, then the ABS warning light comes on and the pedal becomes soft and the pedal sinks to the floor then the brakes lock up until you release the pedal. At that point the car is in non ABS mode, and the ABS warning light is on, a check of the lights gives code 77, and a check with a consult 3 says control unit 4, but no other explination. Does anyone know what Control Unit 4 is? Code 77 says Control Unit/Ground Curcuit and says to follow diagnostic procedure 2 in the FSM. After performing DP2 everything checks out good. The strange part is once the key is turned off the problem, the code, and the warning light goes away.

The feel of the pedal is extremely hard when the problem occurs, and you can't hear the sound and feel the pedal pulsing under your foot, like you would when the ABS is functioning, in this case the pedal is just rock hard, it almost feels like the pressure is not getting to the calipers and is just blocked off. Normally when the ABS would activate it would pulse the brakes stopping them from locking up, but the pulsing doesn't seem to happen, and at the point of the the hard pedal there is absolutely no brakes.

Is this a know issue, has anyone else experienced it, and is there a fix?