There was a known problem with the early style rectangular tipped sensors originally used on the '89 740's. If you read the FAQ paragraph on ABS Sensor Replacement just above the one Steve referenced you'll see a note based on my personal experience: "If suggestions of cleaning the front ABS senders (sensor and pulse wheel) and associated electrical connectors doesn't stop the intermittent ABS fault then you may need new sensors. I found the original old style sensors (rectangular tipped) on my '89 were over-sensitive, especially at low speeds on very smooth surfaces. Your car may or may not have received the in-warranty upgrade to the less sensitive new style sensor (diamond shaped tip). If not, it would be a relatively easy DIY upgrade."

Sound familiar? Your problem seems to be getting worse with age. Perhaps you also have a poor electrical connection or damaged wire as others have suggested.

If you have the original style and decide replacement sensors are too pricey (like >>$300/pr) then go find a set of diamond tipped sensors from an early '90s 700/900. The sensors should be okay as long as they're not damaged. As noted in the FAQ, soak 'em good in PB Blaster (or the like) well prior to removal to make it easier to get them started as they're usually frozen in place until you "crack" the bond after which they twist out fairly easily.
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Dave -own 940's, prev 740/240/140/120's & quasi-expert only on a good day

http://www.brickboard.com/FAQ/700-900/BrakesABS.htm#ABSPulsesatLowSpeeds

I agree with Rob: either bad wiring/connectors to the sensors or (less likely) bad sensors.

It may well be sensors going bad. Sometimes it's only connections to the sensors, but they are getting old- they've been out in the heat, cold, water, and vibration for over 15 years now.

Since you say they've been cleaned regularly, it may be time to try some new ones, or good used ones if you can find a parts car.

I always try cleaning them first and if htat doesn't keep the light off, the next thing to try is new sensors, as well as inspecting connectors and contacts.

The system works like non-ABS brakes whenever the light is on, so you really do want to get it working again. Possibly you can read the code out of the controller, to help pinpoint the problem. An unused fuse spot in the fuse box is the connector for this car.
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Rob Bareiss, New London CT ::: Roterande Fläkt Och Drivremmar!