Without doing exhaustive tests, you may never know. Could be a LiOn battery nearly dead. There may be a dodgy solder joint, or connection.
1) remove all external devices, USB, Sandisk, eSATA, eSATAp, dongles, etc.
2) keep the battery charged if it is a laptop, especially before switching on. My Vista laptop is in that state now. The Win7 is heading that way.
3) It might benefit from strong tape to prevent the battery from wiggling, Duct Tape may be untidy but it does hold. I have used Kapton tape because I have some, it is better at being unobtrusive.
4) once it works don't move it too much.
6) while it is on and working, back up everything you value, regularly.
7) which begs the question about plugging-in USB, Sandisks again - one at a time, see if it mis-behaves.