ABS and Brake Light on intermittantly on '98 A4

I am having the ABS and Brake light come on intermittently (no obvious pattern) and then goes off.
I have been able to determine that the ABS is NOT functioning during
the period that the light is on.
The brakes seem to be fine, and I have checked the Brake Fluid and it is fine.
Any thoughts on what would cause some type of malfunction in the ABS system, and then what to do to fix it?
Dave
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check for error codes relating to the abs, could be a faulty sensor,
Steve.

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