I'm not going to read through all the responces because I'm lazy.
Whoever was editing your project with 23.976 footy in a 29.970 timeline made a big mistake, it is that person's fault you're in the situation you're in now.
While 24p footy (please understand that I mean 23.976p for short), it will look ok in a 30p timeline. But when editing, and hope to export a 24p, your frame mapping will not match on every shot, hence the skipped frames you see in the 24p file rendered FROM a 30p sequence.
SOLUTION, stay in 30p, when displayed on any plasma TV, you cannot tell the difference between a native 24p file vs a 24p file image in a 30p timeline (unless the TV has that dumb 'smooth 120hrz warping' on).
NOTE "DNX145" is either 1080 60i or 1080 30p or 720 60p. The number is the mbps, sort of like ProRes HQ, normal, and Proxy. If you were given the proper format to begin with, it would be either [at 1280x720 23.976p] DNx60 or DNx90, 90 being either 8-bit or 10-bit, DNx60 is sufficient.
If you stay in the 720p30 timeline, the format you should have reiceved is a DNx75.
I went from Avid 2.8 to Avid 5.0, I know in Avid 5.0 you can try and modify the timeline by starting a new project in the 720p23.976p timeline and just open that sequence without a problem. THIS WILL ONLY WORK if the person that digitized your footage was digitized at native 23.976p. If it was digitized at 29.970p, you're SOL.
I guess the dumb final question is, is your footage REALLY 24p? Or are you trying to ghetto 24p by skipping frames?