Well there is several issues that could be arising here and one you would need to use a version of Serato DJ that's 1.7.8 or below as support had been dropped for core 2 duo machines after then also you're running a very old and underpowered machine so if you're trying to run any 720p or higher videos you're gonna have a bad time ... Also look into MixEmergency which is far better optimized for video than Serato Video although I'm not sure that your machine meets the requirements for the latest version of that either ... The version of OSX that you're running will also have a major impact on the function of your machine too so I would suggest staying with Snow Leopard or Mountain Lion at the very newest ... Given that that machine is now 8 years old though I would strongly suggest upgrading your machine to something more current that has a dedicated graphics card

That's actually an "integrated GPU" that shares main system memory. In 2009 it was a big performance jump over past-generation integrated GPU's, but it and the rest of that machine are too low-powered to run current versions of Serato DJ and Serato Video.

It would probably perform okay with Scratch Live and Mix Emergency.

It's an 8 year old mac bro time to upgrade. preferably a 2012 macbook pro 15" which has dedicated video card.

Correction 9 years ago...it's practically ancient