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It appears like that the CPU is overheating during the conversion. The higher the CPU usage, the more heat it produces, especially once it maxes out. Generally this is not an issue when the CPU fan is spinning and its heatsink is clear of dust.
Try checking the temperature using a utility such as Core Temp and report here what temperature readings you get and what type of CPU you have (this utility will state the processor model).
If you can get at your PC easily enough, take off the left side panel (or right panel for some Dells) and make sure all the fans spin up when the PC is powered on. If there's excessive dust on the heatsink, use an air duster (example) to disloge it (preferrably outside).