Power up problem Inspiron 24-5459

Had this problem for several months. When I switch on the mains power at the wall socket, my All in One PC powers up immediately without a press of the power button. It first goes to a CMOS checksum error screen. If I click Continue, the pc will continue to boot but very very slowly until eventually it gets to the log in screen. Programs/apps are then very slow to load as though it is the first time they’ve been used. All settings are remembered.

When I power down the pc but don’t switch off at the wall, the pc boots normally when the power switch is pressed next time I use it.

If I power down, switch off at the wall briefly and then switch on again at the wall, the pc behaves normally and doesn’t begin to boot until I press the pc power button. 

Any suggestions as to what could be the problem?