Positive battery cable - hot start issues

Hi All,

For close to a year I've had hot start issues. Car drive and starts lovely when cold but can't restart once engine is hot. 

I've replaced most of the culprits (starter, battery, sensors, earth cables etc the list goes on). 

However, the car starts when I use a jump pack from the engine bay which leads me to believe there is resistance going back to the battery from engine bay. 

I ran a new negative earth from battery to engine block but didn't seem to help and now looking at swapping out positive cable that runs from battery to engine bay.

My question is, has anyone done this before as I know it's a faff to get to.

Any tips or feedback would be greatly appreciated!