Going back to your first post, you stated it wouldn't pull much of a grade, certainly not what was stated by Genie. And then there's the issue of the wheel spinning. I think you have two issues here.
First, the wheel spin when it looses traction. To some degree this is normal on this machine. Even though it's sort of a full size machine, it's still designed very much like a slab machine, a machine that works on a flat slab. Its rigid frame and non oscillating steer axle makes it real easy to lift a drive wheel off the ground on uneven terrain, it's simply not designed for that. The way the hyd circuit for drive is designed they didn't provide a mechanism to engage the wheel still on the ground when in low drive, drive motors in parallel. However, if you get your boom limit switches sorted out proper, if you get in a spot with a drive wheel spinning, you can momentarily bump it into high drive (motors in series) and it should power the wheel still on the ground.
Second, the low grading power. This could be the drive pump has excessive wear, or the relief valves in the drive manifold faulty or not right. The first thing I'd do is pull the steering relief valve (item 16 on parts diagram), and the main relief valve (item 28). Clean the valve cartridges with brake cleaner and inspect the o-rings. If they're flat or look bad, replace them. Keep in mind there's back up rings on those cartridges, don't mistake them for o-rings. Then you need to connect a 4,000 to 5,000 PSI test gauge to the test port on the drive manifold. Operate the steer (either direction) until the steer cylinder deads out, your gauge should be reading around 750 PSI. Remove the allen head cap on the end of the steering relief valve and turn the adjusting screw inside a couple of turns CW. Operate steer and check gauge. You want to keep adjusting steer relief until you see 3500 PSI, that's the setting of the main relief.
If you adjust the steer valve until it bottoms out, and still don't have 3500, remove the cap on the main relief and adjust it CW, see if you can obtain the required 3500 PSI. If you do get it, back off steer relief and operate steer until you get 750 PSI.
After you've done all of this, touch the drive pump and see how warm it is. If it's just warm to the touch, no worries, if it's hot enough to burn your fingers/hand, the pump may be getting excessively worn.