First up. Have you checked and verified that the electrical connections to all 8 solenoid valves on the transmission are good.? No corroded/damp/dodgy electrical connectors or chafed wires.?
Clutch #6 is common to the three speeds (1F, 5F, & 1R) that you mention. It is also used for 4R but you probalby don't go that high in reverse.
Try swapping the complete solenoid valve assembly from #6 clutch with the one from #5 clutch.
If you then lose 3F, 4F, & 3R in the same way that you were losing 1F, 5F, & 1R previously then either there is/was a bad electrical connection or the solenoid valve that was originally installed on #6 clutch is dodgy.
To identify which solenoid valve is which, look for the plate (highlighted) on the side of the control valve. It has a table listing the location of all 8 solenoid valves.