The only way to know for sure is to wait untill the light stays on all the time so you can pin test all of the wheel sensors. Also here is a good one for all you DIYs. I work on Volvos and have had several 850s with bad electrical part of ignition switch do this same problem. replaced electrical part for other problem and ABS problem went with it. Just one more thing for you to think about
beetlebob, do i look for a short, an open, 12volts, common ground on those sensors?
also i have heard that, replace an electical part in the ignition it fixs ABS issue.
but no one has told me about "that part" what part is it?
Regarding your ABS light, I'd suggest you first find the connector for the ABS sensors on each wheel. With connector disconnected, you can compare the readings from each wheel. As a start, if they are all the same (continuity), the wheel sensors are not likely the problem. If you go to the IPD web site, you can see a picture of the sensors with the cable. The IPD folks are also helpful in giving you troubleshooting suggestions. No, I don't work for IPD.
If you are having problems with other electrical things running ie radio,blower,etc,it might be the electrical part of the ignition switch. I put a lot of them on. It is in the steering column on the opposite end from where the key goes in. Somewhat easy job. For the intermitt problem all I can say is to wait untill light stays on all the time. You can have a bad spot in the sensor wiring (usually a front one) that will not show up when tested. If there is a shop close by that can pull codes from the ABS computer when the light is on go there and have it pulled. If it is still intermitt I suggest that you DO NOT shut off the car and let them know that also. Sometime shutting off car and wire is back in contact mode will not set code in perminate place to get. I also replace a lot of computers. have tried to open them to fix(can be done) but they are glued together and one wrong slip and it is broken and can not be sent in for a core. Hope this helps a little.