Why is my MacBook Pro quitting and/or failing to start up? This is an issue that started today. 

Last night I shut down the machine. This morning I switched it on and got the initial sound but not the chime. The sleep indicator light would blink in groups of 3 blinks. I checked the battery. And also tried some tips on the Apple site for troubleshooting a MBP that won't turn on. Anyway, after looking at a couple of forums, I wound up removing and re-seating the memory. Ok, then it works for about 45 minutes, but then the screen went black and again the 3-blinks of the sleep light. 

What should I try next in troubleshooting this machine? 

I have had no problems in terms of performance or other issues til today. The MBP is from early 2008. There was an iTunes update yesterday and otherwise no new software or updates. Also no external devices plugged in. 

However it did have the power supply connected. I did notice this morning that the power supply had come loose from the power strip -- possibly caused by a cat. This is the only thing that seemed to be a variance from usual.

Thanks.

Anyone? Links or tips on MBP diagnostics would be appreciated.
The blinking is a for of POST (Power On Self Test) and can certainly help diagnose the issue.

As most people are saying it is most likely a RAM issue.

The following documents can help diagnose POST messages and various issues related to your laptop during the boot process:


Intel-based Mac: Startup sequence and error codes, symbols
Intel-based Mac Power On Self Test RAM error codes
Your Mac won't start up in Mac OS X (Mac OS X 10.3.9 or earlier)