The fan clutch is a possibility but since you flushed the radiator and it improved that still sounds like it might be a problem. Pop the cap when it's cold and look inside and see if you can see the tubes for the radiator core. If they have crud built up on the ends you probably have a partialy plugged radiator. Flushing alone especialy with the do it yourself flush kits just get the stuff that can be lostened up by it. The real crud has to be dipped in a hot tank and chemicaly broken down then a thin rod inserted down the tubes to push it out. This is done at a radiator shop with the end tanks taken off. Check a local shop and see what it will run , then check and see what a good replacement will cost. While you are at it replace the thermostat. Thermostats usualy are like light bulbs, either good or bad. Rarely will they act intermittent but are designed to stick open if they go bad................. Murphey's Law gets in there and usualy they will stick closed causing the car to overheat.
As far as the hose, it was designed to stop stumbling due to cold air in the winter when the car was cold, it pre-heated the air coing up to the air filter. Most didn't do much good and if you are in a warm climate just get rid of it you don't need any more hot air going down the intake. If you can, disconnect the hot end and route it forward to get cooler air while driving.
Good luck,
Dennis

Don't give up on your dad yet. Those hoses were actually developed by the sinister Bandage/Iodine Industrial Complex; their REAL purpose is to break immediately and cut the shit out of your hand when you're reaching around it for something else.

That hose supplies passive hot air to the airbox thermostat - not under pressure. I live in Dallas, which is hotter that where you live and have had that hose disconnected for years with no problem. My guess is that you probably do have a cooling system problem and not a temperature compensation board problem, since you state your temperature does go down when the heater is on. It may be that your radiator cannot be flushed out and should be replaced. If so, go w/ the four row. I commonly see radiators for 240s on eBay. Another potential problem is the thermostat.

Trev29 is on the mark, the hose is not that important. I live in the "frozen North East" and have never connected mine. As a matter of fact...I disconnect them; tend to cause more headaches than what they are worth.

Also, have you replaced your thermostat ? My 89, was suffering the same. Steep hills and sitting in traffic on "muggy" days. When I replaced the t-stat I was amazed to find that the one in the head was bent ! Never saw anything like that . I put 850 miles on yesterday...high speed, slow speed, sitting in traffic, and steep hils.....very muggy here. No high temperture !!! Also, a good flushing of the system always helps, Not a bad idea to take a garden hose, and from the fan side, spray/wash out the vents in the radiator. Has to be from the fan side (I know...a bit of a pain) towards the grille.
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84, 242GLTi, 167K, 4+OD, in rehab; 89, 245 DL, 257K, M 47, daily driver; 93, 245, auto, 167K ; and other toys.

The hose is the hot air intake hose "needed" to speed engine warm-up in cold weather. You probably don't need it operational in SC.

Your overheating sounds like it is caused by a bad fan clutch. The fan shouldn't be slipping at high temps. If it does, then at a standstill it can't suck enough air through rad to cool engine.
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1980 245 Canadian B21A with SU carb and M46 trans