Trans slips in 1st and 2nd when hot

[littlefarmer]

2000 F350 2WD 255,000 miles. Owned since new. Never once had an issue with the whole truck. Trans is serviced every 25,000 miles. Never had any clutch dust or any metal in the pan when serviced. Out of the blue, I stop at a light and go to take off and it's slipping. Once up to speed, lock-up is fine. Other than slipping, shifts as normal. Trans operates perfect when cold. As soon as it gets warm it starts to slip. No gauge but my IR thermometer says it is 107 degrees and it is slipping. I have a lot of heavy towing miles, but it's never given a hint of trouble. Today is Christmas day so I can't get it to a trans shop to see for any codes. I can't believe it would need to be rebuilt as it's never given any trouble, but maybe it's time? Anyone have any ideas for what would cause this situation? Or is it just worn out?

[jesilvas]

I've got 318k miles but not as heavily used as yours. I never serviced regularly until 300k miles either.
Is fluid level good?

[littlefarmer]

Fluid level is correct. Just serviced the trans this morning. Truck is bone stock.