Long black screen before BIOS, Windows starts just fine (Windows 8 Pro)

Hi Everyone,

I've been having a very strange problem with a laptop recently (a Dell Inspiron N5010). The problem started with a bad processor (Intel i3-380M), which I replaced with a used one (Intel i3-350M). A few days later, the used processor went bad also. I had a spare processor lying around (an Intel i3-370M) which I was able to put in. Now, however, the laptop does a strange thing - at startup, after powering it on, it hangs for about 10 - 15 seconds (monitor doesn't come on, fans and lights turn on, however) before the BIOS flashes on the screen. After this the laptop boots and runs perfectly normally (although it does seem to run a bit hotter than usual). The other strange thing it does is at shutdown - when I go to settings and hit 'Power Off', the laptop will appear to shut off normally, but then the screen turns off and the fan keeps spinning and the lights stay on indefinitely, until I do a force shut down. I've already tried several things, including opening the computer and checking all connections to the motherboard, resetting and updating the BIOS, checking RAM modules, and running the laptop in safe mode to see if it's some service causing the OS to hang at shutdown.The problem still persists, however. Curiously, if I select 'Restart' under power settings, the laptop will shutdown normally, but will restart with the usual long black screen before the BIOS kicks in. Has anyone experienced this problem before or have an idea what may be the cause of it? Many thanks!