for slow/strange POST issue first try replace cmos battery w a brand new one. also try clear cmos settings by reset RTCRST jumper on motherboard. if no change, then test motherboard using spare working 24/4 pin psu to r/o failing power supply.
  PC Specs: Processor : Intel Core i 5 - 2400 RAM : 4GB (D D R 3) Model : Dell (O p t i p l e x) 990 Operating System : Windows 8.1 Pro GPU : Intel Integrated Graphics 2000   Issues: PC restarts several time Its takes 'bout 5 to 10 minutes stuck on Dell Logo then starts   Explanation: Issue no 1: When I starts the computer it shows a black screen for 'bout 3 to 4 seconds and then suddenly closes after 5 seconds it starts automatically and then it works normally but when after booting (before showing windows logo) it again shut down and then after 5 seconds it starts again automatically and then works normally , starts windows and tasks normally like other computers. Issue no 2: When PC starts for second time it takes 'bout 5 to 10 minutes showing Dell logo then continue.   Things I have tried: I upgraded from Windows 7 to Windows 8.1 I update BIOS from (A 0 2) to A10   I don't why this happens I want help. Please help me.   Note: Sorry for my bad English
Computer restarting issue