You should not have to go to the length of replacing a loom.
Establish whether you are missing sparks or fuel when this happens, and find out what the relay does, what colour is it, does it have a fuse attached to the side of it.
It is more likley something electrical as it is bad when it is hot, though it could be coincidence that it got better when it had a loom change.
A dicky coil is a fav when HOT to give erratic problems just as an example, so if not already done so, a group a coil works absolute wonders on these cars.
I would look to see how the spark is when playing up, so that you can hopefully establish whether it is fuel or ignition, does it always PRIME on ignition ?
tabetha