Sure they rob power. But how many times can you overheat your engine until it gives up, and then it has no power at all!
OP; Beg borrow or steal one, and prove your issue goes away.
Then get one with a THERMOSTATIC clutch on it. The only time it will rob power is when you are not ramming enough air thru the rad; like at low speed or standing still.The thermostat will regulate when to drive it and when to slip it.
And forget all the hype about how much power they suck, cuz those numbers are always biased, to high rpm and fulltime on; a situation that would never occur in real life. Well I suppose it could, if you were towing 80,000 pounds up hill in first gear,WOT for more than a minute or two.......:)
Just test it and see.
Back to that fan;
I bet it follows the same power to resistance ratio that windspeed does; namely, that power increases as the square of the speed. If that was true and it took 32 hp to drive that direct-drive fan at 6000rpm, then at 3000 it would be 32/2squared or 8 hp. And at 1500 it would be 8/2squared or 2hp, and at750 idle it would be 2/2squared or .5hp. An electric fan can draw more than that,lol.
At 30 mph when that fan is likely to kick into freewheel mode, your engine may be doing in the neighborhood of 1300rpm in Drive,so less than 2hp being gobbled up; just before the stat kicks it into freewheel mode.
After 30mph, or thereabouts. Ram-air thru the rad takes over the cooling duties. So; as the airtemp thru the rad cools off, the thermostat on the clutch starts slipping the fan progressively more and more and eventually the fan freewheels in the airstream; just like those pinwheels in your garden do.
Forget about hp loss; just get the biggest damn fan you can lay your hands on, and install it with a thermostatic clutch, and into a shroud.
I have never met anybody who did that and then complained about how sucked out his car was afterwards.
Shooot, if it wasn't for the roaring going on under the hood,I can't even tell when it's working or when it's loafing.