Whenever I use the Equip Menu it only allows me to reach the parts I start off with. The parts that I find can't be reached on the menu unless I spam the movement keys. It makes equipping new parts extremely hard.
When you first start the game and try to change parts, the keys are inconsistent. For example, when trying to change the core part you can use the right key to access the Epsilon-98 Core but when you use the left key to go back to the Roundtable Core is results in no core being displayed at all and the only way to access the Roundtable Core is to press up. You are also unable to access Prescript 0-9-0 Core unless you press the down arrow. You are able to press right to access the Epsilon-89 Core though. It's even worse with the Shears Leg. You have to hold both the left and down key at the same time to access it. If you only press the keys then it stays on the Shears Leg for a split second before displaying no leg at all. Also when you get the other parts, they are usually skipped over when you press keys to iterate through the parts. I've also noticed other issues when the scan lines are on such as the Voltine Core disappearing, turning completely gray, Eps outline showing on the save station, and the outline of other things showing up the same way Eps does when the Save Point is activated.
Yeah, OK, I see. I think you're misunderstanding how to use the equipment menu. You don't press left or right to cycle through the options. You press the movement keys in the direction of the part / selection. So to select the top-most option, press up. To select the right-most option, press right, and so on.
As for the scanline issue, what resolution are you playing the game at?