Try the overdrive fuse and relay. This solved the problem on my 240, with identical mechanical specs. used cost about $25. I don't know where it is located on the 740; probably behind the ash tray and fuse block. Which specific relay I have no idea. On the 240 it is reached by removing the glove box and reaching with the left hand toward the radio housing. Instant cure.
Check fuse first, then replace relay.
If that doesn't work, the problem is probably the overdrive solenoid on the transmission housing. More expensive repair, but nine times out of 20, the relay cures the problem.