my MBP fails to boot

My MBP (2012) was sent back to Apple a couple of months ago, the hard drive was changed by Apple & the unit behaved itself. Now I have got a very similar problem the unit starts up, the password screen, type in log on details, then goes into the Apple logo, then I get a whole load of garbage script across the screen & the unit powers down, then goes through the same cycle once again - Any ideas or suggestions anyone? A computer friend reckon it must be the mother board, any ideas? Your help is appreciated, kind regards

Terru