this sounds like a possible failed logic board or processor module. Since the 2.5 Quad is water cooled, first check the felt pad under the processor heat sinks - is it damp or stained? If so, stop trying to use the machine immediately - the LCS has failed!
Next, make sure you have a known good matched pair of RAM modules (these machine are VERY picky about matched RAM - try to use not only the same size, but the same manufacturer and code when possible) installed in the inner most slot of each bank - follow the diagram on the inside of the cover to ensure the RAM is installed in the proper banks.
Next, if the issue persists when you boot the machine after checking all of this, remove the cover and air dam and check the LED bank on the forward edge of the logic board, near the RAM slots:
http://support.apple.com/kb/HT2652
Red lights here are BAD (except for the "cover open LED" My guess is you will have either the "Overtemp" or "Proc" LED lit.
There are sources for the parts, let us know what you find...