Not sure about replacement power switches, but if your PSP is running any sort of hacked firmware give the JoySens plugin a try for the analogue stick. Allows you to recalibrate it and alter the sensitivity settings. Trust me, works wonders. If that doesn't work, you can get new ones off eBay for pennies. I bought a spare one off there years ago when I was having similar trouble. Cost me 99p, free P+P. Then someone pointed out that JoySens would fix the old one without me having to faff about with screwdrivers and it's been sitting in my case ever since.