I've noticed for about the past month, that I can feel the PMIII kicking on and off through the brake pedal rather erratically. I was sitting at Sonic last night, and listened and felt for it to see what it was doing. I noticed that without pressing the brake pedal, it runs in short bursts real erratically. I heard it do 2 short bursts within 2 seconds..  But while sitting in the garage at home (before driving the car anywhere), I can press the pedal twice before the pump kicks on, and it runs normally then.
Any ideas what could be causing this? (Aside from it slowly working towards it's death, lol)

Mine used to do that and shortly afterwards the pump stopped running altogether.  Maybe cleaning the relays would help.

Oh wonderful..
Where are the relay's at? I've got an extra PMIII sitting in the garage, it may be worth trying some parts from it.