Automatic high shifting problem

[JR Purser]

My 93 240sx convertible automatic has started shifting at high RPMs (all gears) and when I let off the gas, the rpms drop almost back to idle (not staying fully engaged) except in 4th gear (stays engaged). If I manually drop it into 1 st or 2nd, the engine stays engaged to the transmission. I've flushed the transmission but it had no effect on getting the trans to shift at normal RPMs/speed. In order to get it to shift up to the next higer gear I have to get the RPMs/speed past 4000 rpm and let off the gas. I tried the self diagnostic transmission procedure (posted by wiwalsh) but I get no flashes after step 15 on the OD Lamp I'm thinking TCU problem..any suggestions?