engine won't start tap tap it starts!!

[Daniel Neuman <dneuman@stars.sfsu.edu>]

Hello All,
        I have a problem with my car (surprise surprise).  Sometimes when I 
try to start it nothing happens.  I hear a click the voltmeter goes from 
~13.5 to ~8 volts. but nothing happens.  Tap the starter a couple of times, 
tap the solinoid a couple of times and it starts. Last night I did this and
when it started I saw a blue flash from the starter area.  When the car 
first did this I thought it was a ground problem but now I don't think so.  
If it was a ground problem why would tapping the starter fix it?? 
        So, do I need a new starter??  Which one is best?? Has anyone tried
the fancy starter Dean sells??
                Daniel 69 2000
                SF CA