I don't know what the output of PSP supply looks like. I have never seen what a "plug-pack" (what we derisively call a wall-wart) looks like for the PSP. I would be very surprised if it has any kind of filtering at all. Usually these things are a transformer, a bridge (if you're lucky) and MAYBE a single cap (it could be a universal input switching supply as well. If so the thing most likely has tons of high frequency component on the DC. Some ceramic caps strategically placed may be all you need. LM78xx devices don't respond at these higher frequencies and will let a lot of this through). The manufacturer places the caps and related circuitry in the device being powered (for lots of reasons). So first, whenever you are using a "plugpack" put some big caps on the input of the supply to the board. 470uF-1000uF is not unusual. Use some ceramics as well. Watch the voltage; don't use a 6.3V cap on a "5V" supply (higher voltage never hurts). Typically voltage rating is under load, so a "5V" supply may actually be much higher than that.

Mike Tripoli