When it gums up...well it gums up. Can be very difficult to sand as you are finding out. Problem is heat melts the resin in the wood and any dust created sticks to itself and to the grit on the belts and edger paper. Like you are doing, I just kept changing belts until I discovered the combination that worked. Along with the Trio, I also have a Squarbuff that I would use when sanding heart pine. The Squarbuff would generate much less heat (it's one big oscillating sander with lousy dust pickup and slow to boot...lol) but it would help me get the job done.

I don't know if you participate at the Floormasters site or not, but you might want to post your question there. You are a professional, so your participation will be welcomed. Folks over there are professionals with much experience sanding heart pine. You can even search the archive with your question and a ton of tips will probably be returned.

Here is a link that might be useful: Floormasters

Thanks for the tip. Ive had others tell me to take everything to 60 and then buff everything really well. Makes me a little nervous. Every pine job ive done has always been no larger than 3" and no more than 400 sq. this one took me by a little surprise.