[~The Jester~]

Don't swap out the springs yet. What Jeff means is that the MSD distributors sometimes don't give you ALL the advance until 3500. You said you set it at 36* at 2000. What might be happening is the distributor isn't done advancing your timing at 2000. It might be taking you up past 40* at 3500. Leave it alone, and check the timing at 3500 RPM and see what it's doing. Like Andy said, check for vacuum leaks too.