My 2000 Suburban K-2500, with 82000 miles, has started to have some very hard shifts when the transmission is warm to hot. Since I pull a 7000 trailer about a third of the time, I had changed the transmission fluid to synthetic at about 55000 miles. When the hard shifting started, I had the filter changed, and the valve control body. But the transmission started to slam its upshifts when hot again on all shifts, 1-2, 2-3, 3-4, and lockup. Any ideas?