Engine runs fine but seems to seize after wot

So what did the trans fluid look like when you changed it? If it wasn't burned maybe there was a bunch of metal in the filter. If not, maybe you should look to the engine. Is it full with the proper oil? Pull the plug and check it for metal particles. Strain the oil and look for solids. You could have the trans and engine oil checked by a lab. That would give you an idea of the composition of the parts that are wearing.