Greetings wise wizard of EE:
I'm helping a friend with an Mac Pro desktop computer, in which he replaced the spinning hard drive with a solid state drive, running Mojave OS. The computer will very randomly not boot up, instead produces the "prohibitory" symbol (ie: the "ghostbusters" strike through without the ghost). The odd thing is that even when it does boot up normally, it will randomly crash and produce this symbol again.
The operating system has already been reinstalled, so I'm not sure what else can be done. Any help would be greatly appreciated.
Many thanks and take care,