Diagnosing problems in Inspiron 5000e

I have a laptop, Inspiron 5000e and have the following problems.  It shuts off by itself at various time, it will not shut down when I want it to, it freezes up occoasionally.  It was diagnosed by dell (after doing a test whereby you insert the diagnostic disk and tapping f12 it runs for about 4 hours and tells if the computer fails or passes) to be the hard drive and so I replaced the hard drive but still have the same problems.  Now they say it could be the motherboard, the motherboard cmos battery or the IDE controller. I printed out dell's directions for replacing the motherboard and am going to attemp to find the battery and replace it and if this doesn't do the trick will attempt to replace the motherboard.  Also, is the motherboard called the system board?  If it is not the cmos battery, where can I get a motherboard?  Dell said it costs $430.00  Thanks, Rita