Turns off randomly when inactive & twice chime sound when turning on

I recently changed my SSD hard disk (according to instructions on Ifixit), to the respective OWC model, since my old one was failing more often than not. At the same time I upgraded to Mojave 10.14.6.

Ever since my computer turns off randomly, but always only when sleeping/ inactive and once I turn it back on, it does not only take about 5 times longer than before, but it actually does the chime sound twice until the apple logo shows up, so it seems to turn on twice in a row.

Otherwise it seems to run fine (apart from heating up quite a bit, but it did that before too).

Thanks in advance for any advice!