You just changed the transmission fluid and you're already down half a quart? That sounds like a major leak. I'd stop worrying about shifting for now and fix the leak.

Bypassing the BFS didn't change anything and you're not complaining of harsh shifts, prolonged shift transitions or flaring (engine revs between shifts) so I'll assume the transmission vacuum signal is fine or at least not the major problem.

The transmission doesn't have an overdrive gear so I'll assume you mean fourth gear.

There is a pressure control cable between the engine throttle linkage and transmission. This cable is how the engine tells the transmission about throttle position as a surrogate for a load signal. More load = more throttle, less load = less throttle. The cable can stick causing an unsuitable signal to the transmission for the condition resulting in unexpected shifting or unexpected gear selection. If the sticking is intermittent, unexpected shifting behavior will be intermittent. Follow the cable from the passenger side of the transmission to the throttle linkage ahead of the oil filter. Release the ball and socket connection and check that the cable moves freely and smoothly in its sheath. The cable end has a threaded asdjuster. The basic setting is no slack or tension in the cable with the throttle at idle position.

You might try driving with the pressure control cable disconnected. After disconnecting, push the loose end as far into the sheath as it will do. The transmission will shift through the gears quickly regardless of throttle input. You can engage lower gears using the shift lever if you have to but it's safer to stay away from traffic. It might tell you something if the transmission still hangs in third gear.

The slipping worries me since it has nothing to do with external controls and signals. It might be a problem with externally accessible pistons or worn clutches. How long does it take to engage first gear and reverse when you shift from neutral?

Sixto
87 300D