whoa... man sorry to hear... its so weird how there are problems with 4 mac pros in a row!! Only thing to say to you is just keep on exchanging until you get a non lemon and make sure to buy applecare!!
I went through a similar ordeal with the iMac G5 systems. After that, I sold off everything Mac (since they were all failing), and waited quite a long time (almost 2 years) before buying another one new. I got my Mac Pro in August 2007 after seeing that complaints were rather low for that model at the time.
And, I got the other machines I was using in the mean time from other individuals instead of buying them new. They were G4 models that I used to get by until Apple produced a new machine that was reliable.
It would appear that they may be returning to their low quality production model.