Hi, Jonathon.

The same thing happened to Ulysses and here's the explanation they gave then:

Latest on the "Mystery Force"! (16 October, 1998)

A possible explanation for the mysterious "slowing down" of Ulysses and other interplanetary spacecraft has been offered by physicist Jonathan Katz and astronomer Edward Murphy: Ulysses, Pioneer 10 and 11, and Galileo all use radioisotope thermoelectric generators (RTGs) to generate their electrical power. Excess heat from the RTGs is radiated into space, mainly in a direction away from the Sun. The amount of radiation emitted could account for the "push". [Extracted from New Scientist, 17 October, 1998].

http://helio.estec.esa.nl/ulysses/whatsnew.html

IMHO, it's either the above or The Pioneers and Ulysses were actually at the crossroads where interstellar wind meets our sun's solar wind in the area known as 'termination shock'. Last I read, they didn't think that the Pioneers had made it out that far yet. Since 'it' also affected Galileo, then it's most likely the explanation given above (IMHO).

Marcia