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It might be overheating a component, and then force shutting down due to the fail safes that kick in when an overheat is detected.
It could be dirty inside, and that can cause it to overheat very easily.....Can you BLOW IT OUT (meaning blow out the dust) with some compressed air? dont use a vacuum cleaner, you have to use air from a can, or an air compressor.
Ahh, okay so we are on the right track.
Its is possible that one of the cooling Fans has failed (a common thign to fail) which keep air circulating through the machine to keep it cool.
I do NOT reccommend you (a novice computer user) attempt to replace a computer fan, as it is quite a complex task to take apart (without breaking anything) then replace the fan, then put back together(again not breaking anything).
Not to mention, you have to figure out WHICH fan has died, as computers sometimes have more than 1.
I reccommedn you are in dire need of your computer to be repaired.
A computer cooling fan is quite cheap, its the LABOR to fix it, that can get pricey.
Maybe you have a gifted nephew or sibling you can favor to do this?
Or, you CAN send it to me (i operate my own computer repair shop).....but that is on the pricey side, as you need to pay shipping BOTH ways.....labor of $59/hour and any parts needed.
I can send you an estimate if you like?
But honestly im pretty sure we are on the right track that your machine is overheating, and thus shuts down as a failsafe.
If you have enough time on the computer, you could check the EVENT VIEWER, and see WHY it is shutting down.
If it is because of overheat....it should say so.....but you gotta know where to look.
Typically it is in the SYSTEM group of Event Viewer messages. FYI