Problem with downshift 4th-3rd.

[Jan Sentyrz]

I am the owner of the pagoda 250 SL, 1967 In a situation where, while driving along a straight line, the gearbox should keep
4th gear, there is a too quick reduction from 4th to 3rd gear each time the accelerator pedal is released. Has anyone had such
a problem? Does anyone have any suggestions on how to fix this? Thanks in advance for each tip.