Mike, set your pt to interval mode where it displays the total average. Use that for your intervals. I used to set my SRM to display current and average wattage and cycle between those. That way, I could tell if I was around where I needed to be every few secs, and then the rolling average would show me if I was off in total.

Start your intervals lower. Try holding 200W for 20 mins. Then 210, etc etc etc. That should help you get used to maintaining a constant output.