Clutch pressures are in the 160 to 180 range as you describe. There is a guage on the dash to show that. Even when the machine will not move on startup clutch pressures are in range. The brakes do not work but the declutch pedal will take the transmission out of gear.
I think the problem is more related to the torque converter side of the transmission. I would describe its operation as sluggish and it takes about half throttle to make it really move. I have an old michigan 75a that responds to throttle much better than this machine. Shifting is normal and I can feel detents though probably worn as can be expected.
I think the suggestion of a suction leak or collapsing suction line are possibilities. Any suggestions on determining a suction leak or collapsed line? When I crawled under there they all seemed round and hard with age no bad ends or leaks from those hoses. It will be a healthy expense to replace all the lines in that circuit cause they are 1.25 to 1.5 inches in diameter so I don't want to go throwing parts at it.
I will give it a closer look tonight. Maybe drain the transmission and check suction hoses closer.
I remember one day this fall I drove down a steep hill and could not get back up it till the machine warmed up a little more. I gave it full throttle and it could only go up half the hill till about the 3rd try.