Mac OSX not showing up in Boot Option

I posted a question about this before and did not get any responses so here is a shorter version. I'm running a Mac Pro early 2008 desktop with Windows 7 on one hard drive and Mac OS on another (2 seperate hard drives different slots). Again these are seeratr hard drives not a partition. Anyways everything is running smooth and i downoaded the bootcamp drivers for windows to make everything run good on the mac hardware. When I start up my computer though, it will usually start in windows and when i start it up in boot option it will only show windows. Now once in a very long while it will show Mac on the boot option. I reinstalled the drivers and tried to download everythign and then it was working again, when i started up it would show Windows, Mac, and the recovey, which was great and it even showed mac hard drive in windows 7 and boot camp control panel. This was working for a while but then suddenly it stops and it doesnt recognize mac hard drive. I tried resetting PRAM and it did nothing, but i noticed when i reset the SMC it would go back to working perfectly as before and all drives would show up. But then a while later or a couple restarts later the problem would come up again and Mac HD wouldnt be recognized. So I'm wondering what could it be do i need to repair my SMC or PRAM battery or something? Any info would be great, Thanks!