722.5 trans 5th gear issue

[Rital24]

Hi everybody!
Here is Johnny from Italy with his bad english!


I've this strange issue on my 1990 C124 300 24v supercharged with a 722.501 AT.
When I drive the car in "smooth mode" all the gears are working fine but, under partial or full load, the 5th gear takes about 4/5 seconds to engage even if I completely release the accelerator at, for example 180 Km/h.
This behavior is very annoying! The trans has completely rebuilt last year, tried to increase or decrease oil pressure, changed overload protection switch that I suppose has something to do with shift retard but nothing, the problem remains.
I think the switch near the lever is OK as well as the solenoid inasmuch if I don't accelerate to much the 5th gear engages properly.
Maybe the problem stays in the positive pressure in the manifold during boost that could create bad signal to some sensor or EZL unit?

I hope someone of You can please help me on this.
I really appreciate any suggestion!