Faulty Logic Board? Feedback?

Hello,

Last year I acquired this iMac from a family that could not fix the fans running at max speed every time they turned it on. Eventually I found a work around using smcfancontrol followed by inputting a couple lines in Terminal to decrease the fans to low. At first I needed to write these lines every time I booted the computer. But spontaneously during one boot, they did not come on full blast. This problem stopped hereafter (I no longer needed to run the terminal lines).

My more recent problem started a couple weeks ago when the Finder started acting odd. With no other obvious apps running (nor any odd processes viewed in Activity Monitor), the finder would stop responding to any buttons pressed or it would stall on the pinwheel. At this point I was able to shut down and start up the iMac with no problem.

Thinking this may be a hard drive issue (I had similar problems resolved on a MacBook after replacing the hard drive), I installed a new hard drive and successfully installed the OS.

The finder problem STILL persisted. And now it spontaneously shuts down and does a boot loop. It sometimes will boot to the desktop (rare) and more commonly shut down right before or soon after the chime. It has a higher chance of booting after I have left it for more than a day unplugged. Last night I booted it in to safe mode and ran a memtest resulting in no RAM problems. I also took of the bevel and viewed that all four lights on the logic board come on (when it properly boots). Only the first three will come on during times it does its boot loop (crash before the screen comes on).

It is NOT the plug-in (tried multiple). I replaced the PRAM battery. I reset the SMC. RAM tested OK. HD is new (and installed properly). I have no idea about the optic drive, but I don’t think that would cause any problems.

I’m thinking it may be the power supply? Or perhaps more likely, a faulty logic board? The problem may have been caused by the fans not being regulated properly (overheating issue?).

Any thoughts? Any feedback would be appreciated.

Cheers,

Graham