"The early analysis indicated that the anomaly was constant over time. This suggested some force external of the spacecraft was “putting on the brakes,” not an on-board system that would slowly be running out of power. With the newly recovered data, Turyshev and colleagues were able to discern that the anomaly was, in fact, decreasing over time, consistent with decaying thermal power on-board the spacecraft."

http://www.planetary.org/programs/projects/innovative_technologies/pioneer_anomaly/20110722.html