yellow light flashing Optiplex 960 power Misbehaving.

yellow light flashing Optiplex 960 Tower PC But after on and of from wall brick i can make it start somehow i don't know how and it run like a charm and after i use it i shut down and again on it it starts again no problem but after some time like next day it again do that problem i recently made some changes i installed graphics card but before installing the card the problem was not that much intense but there was a problem i think this is a power supply issue and i think i is not enough to run graphics Card that is why it wont start the pc but when i start the system there is no problem then im going to be crazy Please help me i don't know what to do and this is my first question i ever asked on internet on any matter you can think how intense the situation is that make me write this question

Specifications :
optiplex 960 Tower
Core 2 quad 2.66 Q8400
4 gb Ram DDR 2 800 MHZ
2 Hard Drives
Nvidia Quadro 2000
1 Cd Rom
Dell Original Power supply 305W MAX
Anxiously Waiting For the solution