Volvo Repair: intermittent ABS on 1996 Volvo 850(130k miles), wheel sensors, snap on tools

Question
Sometimes the ABS light is off and the braking system seems O.K. But most of the time the ABS light is on and often (not always) when turning and slowing down to a stop at the same time, I feel a shuttering in the break petal and a decrease in petal resistance.
My mechanic said that Snap-On Tools didn't have the part yet for his diagnosis machine.
Thanks,
Stanley
ps I did find an old work order where the previous owner turned it in for an ABS problem while it was in warranty. I've been living with the problem for years because I was told it was OK to drive....I'm having serious doubts lately since the shuttering is more frequent.  

Answer
it will need to be looked at to see what diagnostic trouble codes there may be in the control unit for the ABS system.2 problems I have seen bad control units for the ABS or bad solder connections for the front wheel sensors in the harness.there was a bulletin about redoing the solder connections.