The random shutdown problem is thought to have been caused by faulty memory. If various roms use memory in different ways, one rom may avoid a faulty address space, where another rom may not. That would account for a particular rom apparently solving the problem.
My first captivate had the random shutdown issue. I kept it for around three months after the problem first showed up, trying various things to see if I could fix it, including installing several roms, but nothing I did ever solved the problem. I eventually had it replaced under warranty. The new phone has been golden, for about 7 months now.