4.3 vortec transmision compatability

I have a 94 gmc jimmy 4.3 4 wheel drive my problem is when i first start it up it shifts fine till i hit 3rd gear then it stays in third gear untill i shut the ignition off and resart it. Then it does good till it hits third gear again.
anyone have any ideas what it could be? It's not showing any service
engine light could it be the ignition switch? thanks