Freezing Macbook Pro. Graphical glitches and occasional 3 tones on start up. Hardware test shows no errors. Is it RAM or a graphics card on the way out, or software?

Recently my MacBook Pro has been freezing and showing bad graphical glitches. Multiple restarts eventually fix it and a couple of times the three tones break, three tones break warning has occurred on start up. From what I can find this is usually a RAM error but after doing a hardware check I get the response all is OK. This still happens after the check and I am unsure what the problem could be. Can anyone help? Could it be a memory issue, a hard drive issue, Graphics card issue (drivers are up to date) or is something up with the software. I also have a Bootcamp partition running Windows 7 too and this is a problem that presents itself there too.