Help, Macbook Pro 15" boot problem

Yesterday I went to wake my MBP from sleep mode, and it would not wake. I done a hard boot by holding down the power key, and the machine took about 6 minutes to boot.

Every time I boot now, it goes to the apple screen with the progress wheel for about 1.5 minutes, then sends me to a blue screen for another 4 minutes or so before booting into my desktop.

After spending half the day on the phone with Apple, I have reformatted my machine and am still facing the same problem. I have reset the power (removing the battery and holding down the power key for 5 seconds), reset the PRAM and nothing seems to help. The machine seems to operate fine once it boots.

I ran the diagnostics (holding down the D key while booting) and it says that no problems were found.

Anyone have any ideas I can try. I am about to the point to take it in for service.

Thanks in advance.