87slant6

I have a 87 d150 with a slant 6 and a manual transmission. When I first crank the truck I get maybe 1 to 2 sparks then nothing. Hooking up a volt meter shows its getting power to the coil at first but something is killing the power when its cranking. I ran a jumper wire from the battery to the ignition coil and it fired right up. Once it's running you can take off the jumper wire and it will stay runnin . Only loses power when its cranking. Is there some kind if safety switch that would be doing this? Thanks for any help