580c case backhoe

on my 580c backhoe my shuttle shift isnt working correctly. when you put it in reverse it is right there,and when you put it in forward it wont engage, unless you rev it up a few times, and it goes in and stays in. you can move it from forward to neutral and back to forward and it jumps right in, you can do this several times as long as you dont wait more then 10 seconds. if you do wait more then 10 sec. you will loose forward again,but reverse is still right there.took the pick up screen out and cleaned it,and put in new fluid,checked the linkage,everything is tight,and switches have been all bypassed.any helpful info would be great,thank you tim