1995 GMC Jimmy truck start when it wants to

I have replaced the whole distributor with another one I started to to replace the whole engine wiring harness because ever since I play with harness the truck won't start at all I have replaced every thing at least 3 times that has to do with engine getting spark. The harness is the only thing left i'll keep in touch

There is power at the coil when it don't start. I also noticed that when it ran the transmission was not shifting right sometimes and it would missfire sometimes when engine got warm. I'm going to get wiring harness from junk yard. I think that one of the connectors at computer are bad on old one will find out soon enought this truck has been to 3 different mechanics including the dealer none of them could fix it and i'm not paying the dealer to put new wiring harness in. Puting in the harness dosen't look fun it runs throught out the whole engine comparment and all underneath if you have any tips on doing this please let me know thanks for your help

Replaced wiring harness truck runs better than it ever did it sounds much smoother not a fun job but I got it done. The idle is up high I think I have a vacum leak somewhere but it run good