I'm assuming the one you have is the version 2?
I've built a number of these machines for others..... the two areas that caused issues, and I had to modify are..... the pivot/joint where the idler arm bolts to the frame/upright. Using a common bolt there allows for "slop", and allows the idler arm to move slightly side to side. On a couple of the machines I installed a "yoke" on the frame/upright with close tolerances that prevented the side to side movement. On a couple of others I bored out the pivot hole in the idler arm and bushed it, then used a precision shoulder bolt to eliminate the side play. Any bolt with threads at that location needs a nylock nut on it.
Although a lot of folks think using one of the "spring cylinders" for belt tension is the cat's meow, I simply don't care for them....particularly on the GIB type machines.
Spring cylinders don't care what belt you have on the machine, nor how hard or soft you're pressing against it...... it applies the same force no matter what....which in the case of varying belt types isn't always a good thing. That being said, I do have a spring cylinder on one of my grinders, but that because that machine never runs anything other than an X weight belt on a 10" wheel for profiling. If I put anything lighter than a X weight belt on it, the belt jumps all over the place when grinding. On all those machines I've built for others, I deleted the spring cylinder and replaced it with a coil spring.
The variable rate that a simple coil spring provides keeps the tracking constant, no matter what belt you're using, or the amount of pressure you're putting on the belt. In my experience, with the spring cylinder, Y and X weight belts run fine, and J or finer tend to jump around. With a coil spring, all belts track the same on these type machines, without any jumping around.