Good, This will definelty narrow your searches.
also i would like to say the type of coolant etc wont effect your cooling this drastically if you decide to use better coolants its like putting a band aid on cancer
what type of fan setup are you running is it a pusher or a puller if its a puller thats part of the problem that wont work for **** at any sort of speed.
Here are some possibilties. Waterchannel in block is blocked or theres some type of material stopping it from really getting inside the block and into all the crevises,Timing is off, bad tubing somewhere that slows down the flow considerably Kinks blockages etc, exhaust problems, Sticking injectors
thats just some stuff i thought might cause it feel free to criticse etc