Sounds like you've got debris in your valve body. Probably around the 1st to 2nd detent plunger. It's not hanging up between shifts (feels and sounds like neutral), it's hanging up while still in gear, (not releasing). You stated that you've had the transmission for 6 years and you haven't done a flush yet.

Many years ago, we got away with a trans flush every 50,000 miles. Not so today. We should be flushing every year, or sooner if you drive it hard. If you haven't done a flush in 6 years, that's not the problem...It's the cause of the problem.

Also, you might want to look into the front pump (in the bell housing). The vanes might be worn and not creating enough pressure to operate the valves in the valve body.