Mine did this too.
It happened twice: Once when the fan broke, the other time when the battery wasn't able to charge anymore.
Check I8kFanGUI to see if your fan over the processor is able to work.
Then remove the battery and try using it, see if it goes full speed from there.
If fan, call dell.
If battery, buy new battery.
My Dell did this when the fan broke.
It did it again when the Battery was running out of juice.
Check that the fan over the cpu is working properly. I think the program is called I8FanGUI or something like that.
Next remove the battery, plug it in, and try it that way.