When towing trailer with under 750lbs transmission acts as if in neutral, and spits out fluid through the breather tube

What is your question?

Happened 2x. Runs fine with no trailer. Checked breather tube and it is not plugged up. Would too much fluid cause this to happen? Power washed transmission cooler up front.

What makes this problem better or worse?

Towing

How long has your 1999 GMC Sierra 1500 had this problem?

1yr