Doubt the hose is sucking shut, he says it cools down when driving faster than 45mph on a steady run. I'd bet it's tune related. Only been driven 300 miles in 6 years. You haven't driven it enough to make a difference. Have you flogged it at all? Do you know if it makes any power or runs correctly? Or, do you just drive it around like you're delivering milk? If you can't tune it, take it to someone who knows how. You may well be chasing your tail messing with the cooling system.
Travis..