963 occasionally slows travel

21z 963. New engine. Machine sat for 3 years wile dead beat mechanic installed new engine. I changed trans filter and oil. Occasionally the travel speed will go down to 50% or so. If I idle down and idle back up it will go back to normal for awhile, then back into slow mode. Tried changing filter again, no change. It is a napa filter. Got a filter from cat, but have not tried yet. Any ideas?

963 occasionally slows travel