I can tell you that I also had some weakness at afternoons when I was on Duromine. However, I thought it was due to my stresful work + kids and home routine... Maybe this is one of the effects of Duro, like you know you have energy boost first and then when you do a lot of stuff, it disappears suddenly and you feel this tiredness.
Actually I've noticed that this past three days sitting here now thinking just that and found your blog. Yesterday I took a nanna nap for 20minutes, but I'm working past it today. Good luck.