These symptoms are notoriously difficult when it comes to a diagnosis - there could be any one of several causes.
The fault could be indicative of a failing hard drive, or a dry connection on the motherboard, or a loose cable connection.
If you feel confident about opening the back of the machine (when it is turned off at the mains supply and the battery is removed) inspect all the cables - check to see if there is any chafing on the insulation covering, or any loose pin connectors on cable ends.
Make sure the RAM modules are properly seated, and that there is nothing that would obstruct the fans.
Your access to components is limited with a laptop, but check everything you can see.
Then, just try to avoid jostling the machine when it's running.