simplex try with a single piece of the good ram so you get the same total ram, or some app that fills up the ram.
It is also plausible that the system does some kind of check or negotiation early on if the ram is fast enough to enable some feature. Less likely than it simply being something turning off when the system is low on ram tho.
It is also entirely possible for one ram to run at a multiple of some other process with the good ram, and that changing it causes different things like when a buffer is emptied etc to happen either randomly, causing less stable output, or simply ending up with trying to do something at the same time as something that shouldn't be locked. These types of effects can be seen in other places, for example when it does garbage collection from running out of memory. If that happens when there is clothing going on it is fine, but if you suddenly get it as you are trying to open a new browser window you would get freezing for a little while. Also there is a reason we match video to the power supply frequency.
Just out of curiosity, are you in the US with 50hz or Europe with 60?