Not all memory kits will run at rated speeds on all CPUs since it's basically an overclock. They usually have a disclaimer on their website as well. As you found, reducing speed or loosening timings can help. Otherwise it's a process of tediously testing sub-timings and voltages to find a combination that is stable. You can try changing your kits around. The CPU that works OK with 4 sticks might work even better with 2 sticks, then you just have to figure out what is stable on the other system when you increase it to 4 sticks.
Sometimes you can use a RAM calculator to give you a head start on the manual tuning but I personally haven't had much luck. Kits that didn't want to cooperate just kept crashing.