whoa, wait, have you been running the entire time with no shroud? that could be it right there...

also, no, 5k rpm is not normal, at 80 mph, the normal rpm is under 4k...

and are you sure you're not running lean? are you running a k&n & hi-flow exhaust? you might be running lean if you are...

also, is the fan sized right for this application? i've seen the size, but i mean cfm's...

the deal with the shroud is that it forces air to come in from all over the front of the radiator, helping to cool all the surface area... without one, you just suck air thru the center...