[fxbill]

If it has been some time since you've done any cleaning in that area I'd take a hose with reasonable pressure and hold it up close to the radiator to flush out any accumulated junk that has possibly lodged in the cooling vanes. I've had the same problem with high speed cooling being less than at low speeds and it had to do with the airflow at higher speeds. Also check your hoses to make sure that your engine intake hose is not collapsing at higher RPMs. If it's ribbed or has a wire coil in it it should not be a problem, but I've seen it happen before and it's an elusive problem to find....at normal speeds eveything looks OK, but when you're on the throttle the hose will begin to collapse and close off the water flow.