Volvo: ABS light on randomly, throttle position sensor, mode selector switch

Question
I have a 96 850 GLT and have replaced the ignition switch, throttle position sensor, and the mode selector switch within the past year. Yet, I still sometimes see the ABS light come on when driving at slow speeds or stopped.  The dealership has never directly associated the ABS light with any specific thing because no codes come up when they test it.  It wasn't until I went in for a check engine light (which I attribute to a bad tank of gas...I hope) and mentioned the ABS problem, that they told me the control module was bad and needed to be replaced ($$$)as well as another expensive part/labor for replacing a part of the emissions system.  
Could all of these problems be related?  Why all of a sudden is my car showing codes for the ABS and not at anytime before?

Answer
Jamie,
The ABS modules on 96 and up 850's has had many problems and more than likely will need replacement at a cost of $375.00 for the part plus the labor. As the control unit goes bad the light will go on and off until the unit is bad enough to set a code that can be read.
I do not know of any reason that the two problems are related. You have the emission and then a brake problem, seperate issues.