What should I check if my 485 Quadtrac won't shift properly while pulling a direct-mount scraper?

[Gryks Machines]

I'm looking for some advice on a transmission issue with my Case IH 485 Quadtrac. The tractor mainly pulls a direct-mount scraper for drainage ditch and earthmoving work. Lately, I've noticed that the transmission doesn't seem to shift properly when the scraper is loaded. It has been calibrated a couple of times, and the shifting improves for a short period, but the problem eventually comes back. It almost feels like the transmission is slipping during gear changes, especially when pulling under load.

Before I start replacing parts or spending more money on diagnostics, what are the main things I should check? Could this be related to transmission calibration, sensors, hydraulic pressure, clutch packs, or something else?