[Bill Baer]

Well the object of using a relay in the neutral safety circuit is to reduce the amp draw and arcing in the relatively small micro switch in the shifter. The best way to wire it is in the aerospace components shifter instructions available on their web site.
However the problem is probably a poor ground. I run my primary ground wire to the starter because the starter circuit draws the largest amp load in the whole car ( more than 800 amps!) and if your ground path won't consistently handle the load the starter can't perform as designed.
A simple way to test for this is with a pair of jumper cables clamp the negative cable to the battery negative post and the other end to the starter housing and see if it starts better.
If you using the body of the car or the chassis and or motor plates as your ground path there are a lot of places were you could have a bad or intermittently bad ground.