Optiplex 780 has an inconsistent problem

I am a student at an IT networking school.  Each of got a PC to work with and with my luck I got the problem one.  At the beginning of the year it was working fine when all of the sudden it began to act up.  The computer was very slow to get past the BIOS startup screen.  I checked the system configuration and eventually removed the CMOS battery and fixed the problem.  After that it worked for a while.  Then it got caught in a startup loop.  Sometimes I would get past the BIOS screen and sometimes it would bring me to a blue troubleshooting screen before restarting a few seconds later.  Eventually I reinstalled Windows Server 2012 RS.  When I first loaded it up it gave me a blue screen with a dump file listed.  I exited out and searched around but wasn't able to find much.  The next time I started up it gave me an amber power light with blinking 1 and 2 indicators.  The strange thing is it does not always do this.  Some times it starts up and works fine but some times I get the amber light and blinking indicators.  I've looked around but I haven't really found anything.  I think it is likely a loose connection since it only gives the error some of the time.  I have no idea what is wrong and I could really use some help.