I wouldn't have said yours looks like a sensor problem as it is so random, one minute there's 2 playing up then it's the other 2 playing up. Which line is the battery voltage?
Try cleaning the plugs at the sensors, also the plug at the abs.
May be some water ingress, may be worth while changing the battery and getting the alternator checked too
Instrument panel connections:
The instrument panel E50 is connected from pins 3 and 4 of connector A with the Body Computer M1 and via the latter to the entire CAN: the signal reaches the electronic speedometer (from the ABS control unit M50) via the CAN: the same signal is replicated by pin 16 of M50 at pin 7 of connector A of the Body Computer M1.
The electronic rev counter receives the engine rpm signal received from the engine management control unit M10 via the CAN.
ABS module conn.:
The ABS electronic control unit receives a power supply (at pins 12 and 25) directly from the battery from the line protected by fuses F5 and F4 of the engine compartment junction unit B1.
The ignition-controlled power supply (INT) arrives at pin 23 from the line protected by fuse F42 of the junction unit under the dashboard B2.
The control unit is earthed at pins 11 and 24.
The four sensors K70, K71, K75, K76 send wheel speed signals respectively to pins 19-20, 7-8, 21-22, 9-10 of control unit M50.
The brake pedal switch I30 sends an enablement signal to pin 17 of the control unit M50: all intervention of the ABS is excluded unless the brake pedal is pressed (signal from N.A. contact); switch I30 receives an ignition-controlled power supply (INT) from fuse F37 of junction unit B2.
The "discreet" speedometer signal sent by the Body Computer M1 leaves from pin 16 of M50; the same signal is replicated via the CAN.
Via the CAN line, ABS control unit M50 is connected - from pins 1 and 2 - to the engine management control unit M10, the Body Computer M1 and the instrument panel E50, to manage the ABS warning light and, if the problems concern the EBD function, the low brake fluid level/handbrake applied warning light.
The autodiagnostic data can be read by connecting to connector C of the Body Computer M1 - pin 1: it receives signals from pin 6 of control unit M50 via the special diagnostic line.
Hope this can help you :-)