overheat on 96 accord

Hi, all. 1996 Accord EX-R Vtec 5-speed 320,000 kms
I have a somewhat perplexing overheating problem. Starting last summer, I noticed the temp gauge would slowly rise at highway speeds with the A/C on
and while I was pulling a heavy boat (it didn't overheat for 4 years previous with in this condition). I shrugged it off as being something I should expect to happen under those conditions. To make a long story short, as of now, on moderately warm days, the car will overheat with just me in it, no boat no A/C. The temp gauge climbs only slightly, but putting on A/C, climbing a hill or obviously, pulling the boat worsens it to the point where I need to throw the cabin heat on full to keep the needle from climbing.
This is what I have done/noticed during the past year. The rad seems to be in fine condition ( I've had worse) and the coolant always looks as new. I took it into Honda and they diagnosed the sensor that turns on the fans (the same two fans that the A/C opereates but this switch turns them on when the car is running even if the A/C is off) as faulty. (I had already determined this as the first time I noticed a severe overheat condition was when I left the car running for 4 minutes to get a coffee. After returning, the needle was all the way in the red. If I hit the A/C button, the temp dropped rapidly when the fans kicked on.). Being a student, I couldn't afford the $50 for the sensor so what I did was short-circuit the switch, making both fans run full time. The condition persisted. I was convinced the entire time it was my [original] thermostat but Honda told me it was fine. I changed it anyway and Honda was right, as the problem is still there. Bottom line is, it's not the thermostat and the cooling fans run al the time and the car overheats.
I have read on these N/Gs that there are often faulty temp sensors. I'm sure Honda checked this out. I also believe this to not be the problem as the symptoms seems to point to an actual high heat condition. So what are my next possibilities? The only two I can think of is restricted flow through the cooling system and/or a rad that needs replacing. Can someone recommend to me how to check for either? IS there such a thing as a cooling system flush that removes built up residue? Advice on changing a rad?
Thanks a million in advance, MajBach
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MajBach wrote:

if you've eliminated the sender/switch already, that's obviously not it. likewise the thermostat. next candidates are head gasket, bugs in the radiator matrix, or lastly, internally blocked radiator core.
head gasket should evidence bubbling from cold with the radiator cap off. bugs can sometimes be removed by flushing with lots of water, but avoid excess pressure jets as they can ruin the matrix elements. if its none of those, it's likely a blocked core. those can be cleaned by flushing, but i don't liek to do that on my own cars in view of the longer term effects the chemicals can have on the rest of the system. replacement radiators are not wildly expensive, so my preference is replacement rather than flushing. ymmv. my money's on the bugs.
when refilling, use distilled water with the antifreeze, not water from the faucet. also, take care of the little things like checking ignition timing.
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