can I buy a new hard drive and load my recovery operating system on it?
Yes Bob, you most likely could but it would be a 'restore to factory settings' recovery which is a destructive method. All updates, installed programs, settings and configurations would have to be applied over again - which is a heck of a chore.
Best method is what Marko suggested. Use something like Macrium Reflect free to create a full system image and keep that stored on an external drive. Create a fresh image every month until you have two or three images saved, then start deleting the oldest image each time you create a new one.
I don't think your MBR is damaged or corrupt because if that were the case the machine would generally not boot at all. The fact that you can still boot to the OS on most occasions suggests that a hardware component is on the way out. We could try a few things but the intermittent nature of the problem makes it impossible to know whether possible fixes have been successful or not.
I suspect that the black screen and flashing cursor will eventually become permanent and you will not be able to boot the OS at all. Until that happens, all you can do is take the necessary precautions which, with data backups and image creation, you will be doing.
Couple more questions: on those occasions when the OS will not boot, are you hearing any beeps at all? And how do you then get the machine to boot?