Have you considered ergonomic bars, rather than switching to flat? I haven't been fitted in 6 years (hardly any time compared to you!) and am going for a re-fitting next week because I'm having trouble positioning my saddle, and my hands go numb around mile 25. I'm seriously considering the ergo bars--you don't want to lose to possibility of going into drops, but flattening at the top definitely helps to relieve nerve pressure. You also might need to change your stem configuration--perhaps the 0 degree rise is part of your problem.