A few thoughts:

- Sporadically not shifting out of third. This is probably the downshift switch, which is located under the accelerator pedal. You need to lift the carpet mat for access. Because of where it is, not unusual for it to be fouled with dirt and fluids. Mostly, you can clean them, but it may need replacing.

- Harsh 2-3 shifts. Try replacing the K1 spring in the valve body...search for more info.

- Vacuum amplifier(BUFO). There are four or five hoses terminating on the Bufo. The one for the transmission is the one on the top right side. This vacuum signal is a modulated signal based on the position of the vacuum control valve. If you move your Y to the bottom right connection, you can read the source signal. If the source signal isn't variable, check the connections on the VCV, which is on the right side of the injection pump. There is also a feed from the turbo, and a feed from the vacuum pump.