My shape layer keeps jumping between its pre-expression and post-expression position

You have separated dimensions, moving the xPosition by adding the value of the first slider, then you move the Yposition by multiplying the value of the first slider by the value of the second slider. That means that if the first slider is set to 200 and the second slider is set to 100 X will be transformed from its current x position 200 pixels to the right and Y will be transformed from its current Y position 2000 pixels down. 

I am assuming you have set at least 2 keyframes on at least one of the sliders. You could also have set keyframes for X and Y positions.

The first thing I would do is delete all keyframes for the sliders and positions. This should stop the movement. 

Then I would reset the value of the first slider to 0 and the second slider (slope) to 1. Adjusting the first slider (iso z ctrl) should move the layer diagonally. Then adjust the value of the second slider to modify the Y position.

Your expressions do not give very usable control to the X and Y position. I'm not sure what you are trying to accomplish. You offer no design goals other than moving a layer diagonally so I'm not sure what you are trying to animate. At the very least I would multiply the value of the slope slider by .01 to give you some reasonable control on the Y position.

I suspect that you have multiple keyframes and that the resulting values are all over the place. Personally, I would not separate dimensions and I would use two sliders and a little math to generate an array, then set just a couple of keyframes. My solution:

anglCtrl = effect("iso z ctrl")("Slider");
slopeCtrl = effect("slope")("Slider") * .01;
[value[0] + anglCtrl, value[1] + anglCtrl * slopeCtrl];

In the future, please embed a screenshot of your timeline with the modified properties of the problem layer showing. Just press 'uu', take a screenshot, copy the file name and paste to the forum, or drag the file to the forum, and don't use the "Drag & drop or browse..." feature because it's buggy. Without knowing what is going on in the problem layer it's pretty to offer anything but a guess.

Aside from the same boring answer of "Update and tweak your graphics driver" there's really not much one could say. If you haven't already, also turn of multiframe rendering. Out-of-order rendering can cause weird things when using expressions.

Mylenium

You can also view the video of the issue here: https://www.reddit.com/r/AfterEffects/comments/sj3gwi/shape_layer_keeps_jumping_between_preexpression/