Dirty transmission fuel could cause the transmission’s gears or springs in the governor to delay shifts. You mentioned that the transmission fluid appears to be clean, so this may not be the problem. One of the common problems with this transmission is slipping between the 3rd and 4th gears. Overheating can weaken the transmission’s clutch pack disks, causing the ends to shear off and causing a serious problem. This gear slipping usually occurs between 3rd and 4th, but I would assume that it could happen with any of the clutch pack disks.