Adrian's answer is valid and is looking at it from a potential root cause. But my preferred answer was a work around that got answered in the bitcoin stackexchange

Direct Quote:

Using bitcoin-qt you can go into Helps->Debug Window->Console and then type

setgenerate false

By default it should already be false so my guess is that you have a fast internet connection and that your computer receive a lot of blocks in a short time. Your computer then need to check all the ECDSA signatures for the transaction and this can consume a lot of CPU.

I had an overheating issue like that when I was mining litecoins with my laptop. I installed the cpufreq package to decrease my cpu frequency about 30% and everything was fine then.

If you have a dual-core processor you should type something like this in your terminal.

sudo cpufreq-set -c 0 -u "1.8GHz"

sudo cpufreq-set -c 1 -u "1.8GHz"

I then decide to browser around for CPU Freq scaling and decided to use the following indicator-cpufreq indicator

Computer did not overheat and got the job done without suffering to much performance loss.