If you haven't done it already, I'd suggest cleaning out the fan and heatsink and replacing the thermal paste, as well as buy a notebook cooler stand. Those notebook-bound Westmere mobile processors were made with very low TDPs, and they can be made to max the thermal levels out pretty quickly. The 640LM is also roughly equivalent to a Core i3 processor with very low clock speeds, as it only has two cores with hyper-threading, so video editing won't be spectacular.
It would help if you can tell us what you're trying to do, exactly. 720p and 1080p video should be doable on that system, but anything like 1440p or higher is a no-go.