Power supply problem?

I have a really annoying power problem.About 3 weeks ago,I came home from work,to find an amber blinking light coming from the power button.My Dimension 8300 would not turn on.A few hours later,it worked fine,but not even 5 minutes later turned itself off.Same problem after that,the amber light would blink,and it would not work.I call Dell,and went through some steps to find the problem.The tech told me to pull out all the cables from the drives and cards,and test to see if we could narrow down the problem.Anyway,she said my problem was a power supply problem or a motherboard problem.I had ordered a power supply from Dell(they shipped me 2)installed one and got the amber light again.After switching the power selector in back of the power supply,over and back again,bingo,it worked.Now all of a sudden,2days later my PC will only turn on at times like before,and it shuts off right after loading to the desktop.Today is the first time it has worked in a day or 2. Please help.I use my PC for work and need to get this fixed. Specs:Intel P4 HT 3.0GHz 1024 mem 200 gig HD ATI radeon 9800 pro 128