transmission of my Sierra 1995

First, excuse my english, i'm french from Québec.

Sometimes when i'm going on the road, my Sierra 95 with diesel 6.5 litres engine seem do not change of gear transmission. It seem to stay in first speed.

After this, i stop the engine, re-start and the transmission is OK.

What's wrong!!!!!

It could be your ignition switch, next time it happens check voltage at trans fuse, if it isn't 12V there is a good chance the switch is bad

speed sensor