As the weather warms up we are seeing more and more ABS failures. When the ABS unit gets hot the circuit board detects a fault and the ABS light turns ON.
Once you let the car cool down, the trouble is in the memory and the light is on; however, as you start driving, the ABS unit performs a self test and the problem is no longer there so the ABS light is turned OFF.
Eventually this cycle will get to the point that the ABS light will be ON all the time and the speedometer stops working.

All the ABS control units (Installed on 96 and newer 850, 70 series and S80 models) will eventually fail after they reach 65,000 miles or 5 years. The average is 85,000 miles and/or 6 years for failures.
A dealer will charge from $60.0 to $120.00 to diagnose the problem and about 95% of the cases the mechanic will recommend to replace the ABS control unit.

Here's more information on this issue and a replacement alternative.
Vitor's Volvo ABS website