There actually is a fuse in the computer box, I assume your E420 uses the same as my 92 400E.

That fuse (F1) protects the ABS module and throttle control unit (cruise control), if that fuse is burnt, ABS won't work, you will have ABS warning light all the times, cruise control won't work and engine rev won't pass 5500 rpm, I had all these problems on my 400E, replacing that fuse cured all.

Since ABS warning comes on intermittently, I would guess FUSE is OK, cleaning the ABS sensors will be the first thing I would suggest, also it's a good idea to check the battery voltage at idle, it should increase after you start the car, from something arround 12.3 to 12.8 Volt.

__________________
99 BMW 540i 6-speed 110K Km
03 SAAB 9-5 wagon 80K Km
92 400E (Sold) 245K km
Still missing the days with the Benz, it kept me busy.