i had the same prob when i built mine only 3 months back.

turned out to be the water temp sensor,
under the back of the inlet mani.

not faulty, but the wrong coler plug for it.
from memory, it was a brone sensor, and the black plug made it run fine.
ran like yours with brone plug pluged into censor.

Colers could be wrong, might be the other way round, black/brone.
Just change the plug on censor for the spaer one back there.