since you don’t have valgus collapse with lighter loads, it is likely a motor control problem during stress. when you get stressed with the heavy load, something goes awry and you compensate by valgus collapsing. having said that, it is possible that there are some hidden mobility problems which may encourage such a compensation.

i would work mobility as usual – make sure there is nothing in the hips and anything up/downstream that needs to be cleared up.  can you air squat all the way down ass to heels without butt wink? mobility to achieve that will decrease the chance that you’ll compensate due to some unknown mobility problem.

i would try also squatting with a wider stance. what is the width of your stance now? 

how low do you go on the squat? you may want to decrease the depth of the squat until you resolve this issue.

you can also try inching up to the problem. find the max weight where you don’t valgus collapse. then find some fractional plates to add on only .5-1 lb at a time and see if you can repattern up to the first problem weight.

when you come back up from your deepest part, do you squeeze the glutes and drive the knees outward in rotation with both legs?  sometimes it can be your external rotators of the leg that aren’t firing properly. you have to force them to do it consciously until it becomes natural….