Vostro 1720 keeps switching off

Hello all, I have a Vostro 1720 that runs Win 10, but lately it has been running very slow. I had the HDD replaced by a SSD, the RAM maxed at 4 GB (previously 3gb) and a fresh install of Win 10. Since this was done, by an IT engineer, the laptop keeps switching off and back to the home screen. It doesn't seem to matter what the laptop is doing. This never happened before, even though it was slow at times. I used HD sentinel, no problems with the SSD and memchecj to check the RAM, again nothing reported. I have ruled out the power supply (ie the external cable one). Any suggestions please?