Info found on the UK Volvo owners club website. Could this be the cause of the problem?

Some early 850's sometimes suffer from a pulsing brake pedal - most noticeable when driving slowly in city traffic. This is basically a manufacturing defect: not all the plastic sheathing is stripped off the ABS sensor wires at both front wheels. A factory modification kit is available (under warranty) and fitting requires removing the existing wiring to both front wheel sensors, but not changing the actual sensors. Instead, you solder the new wires on to the loom. The connections are made inside the square plastic trunking on the nearside, near the ABS module. The joint must be soldered, as a crimped connection will cause current drop and affect the system's operation.