Ok, if you didnt take down the entire 2-speed assembly then you need to and inspect every part. With that out the way, then your tuning could be off, most likely lean.
When does this problem acure? If i am reading right, it happens while the car just sit there(idleing). If that's the case, then it isnt your 2 speed cuz idling has nothing to do with a 2 speed. Check out the washers and needle bearing on your pilot shaft. It could be missing a washer or something is worn. INSPECT everything on it. I have the nrs4-2 with the hd 2-speed on it as well. What gearing are you running with your 2-speed. I noticed with the use of a 15/18, 13/18, 13/20 clutchbell i have to add or subtract washers. I also have to use 2 washers to hold on the clutchbell since the clutchbell hole is much larger than the stock clutchbell. Till this day i dont understand why the clutchbell hoel is bigger than stock.....i mean the single washer dosent do jack as far as i can tell.
Wow these forums have gotten popular since 02!