Hey,
One problem i had when i put a sports dash in my esky was, The new cable i ran off the Coil Negative was arcing behind the tacho, wich resulted in MAD missfiring and absolutly no power. Make sure that cable is only touching the right terminal behind the dash.
Also with the 8mm leads. I got some from Repco (8mm silicone) they only lasted me 3 months. The heat from the exhaust manifold broke down the silicone causing them to split and have very weak spark. Wich resulted in me having the same problem your having. Check your leads for any cracks.
Don