Trans heats up at speed, under light tow

[cchicoin]

I bought an 02 F250 for towing some toys and light plowing. Things went great for about a year and the trans cooked. TC burned up and wouldn't lock, causing excessive heat. I had the trans rebuilt and it's been fine for 2 years, not issues at all, until now. I was on my way back from Watkins Glen towing an aluminum car hauler and a stripped down subaru, less then 3500lbs total. At 70 mph, trans runs cool as can be, at 75 things seem fine, at 80, it starts to heat up. If I back it down to 70, the temps go down to normal. I have an aftermarket cooler and TC. I give up, everything works fine and it doesn't slip at all. Any ideas?

[dereku]

Sounds like something is causing heat build up under load.

What kind of aftermarket cooler do you have? A lot of them are worse than stock, best is a 6.0 cooler.

When was your last trans service? Filter could be clogged or fluid could be bad.

Do you have any shifting issues?

Is your fluid low? Could have a leak.

Your cooler could be clogged.

The bypass valve can be bad but those normally cause issues at all times.

Is your coolant good? The cooler that goes through the radiator does more than most people think.

Could have a bad clutch fan.

[cchicoin]

The trans has about 10K miles. Fluid is checked weekly. I have not checked the filter but when I put the new trans in, I did flush all of the lines and removed the lines going to the radiator. The strange thing is it shifts fine. It doesn't heat up when towing up a hill with the TC unlocked but it heats up at 80 towing almost nothing on a flat highway. I'll try the flush and filter and go from there.