88 GT problems

I have an 88 Escort GT that I just put a head on. Its been sitting for about a month.It runs real good. The problem is when I want to take it out on the road and try it out as soon as I put it in rev. our any other gear its a 5sp it just stops running just like you cut the power of the fuel pump the radio all power goes dead . It won't start by pushing in the clutch when it is in gear you have to take it out of gear then it will start wright up.If you take the shifter and just move it around it wants to kill the motor.Is there a switch down in buy the shifter that could be doing this? If you have any ideas would be a great help. Thanks Lanny

Not sure about a switch in that location which would cause the problem.
I would check the linkage from the shifter to the trans while someone is moving the shifter to see if some wiring is being pulled or rubbing on the linkage. If so, Reroute, reconnect and repair wiring as necessary.

The only switch that's related to your shift linkage directly is the 5th gear upshift light cutout switch. I'm guessing you might have some loose wiring.