I was on the NG for 5 years and as I recall..if you fly past the TD point, the a/c goes into energy compensation mode if in VNAV. When you execute the descent now the a/c initially descends at 1000fpm then tries to capture the path. FMA shows IDLE, VNAV PATH. Obviously it cant capture so it reverts to VNAV Speed and ingores the FMC constraints for speed while trying to recapture path mode. Simple ans is use LVL CH and wind the speed up, when the magenta path deviation is above the the line just select vnav again.