Creation of negative float, when end date’s is zero

Hi, I am experiencing problems with Version 5. Primavera are assiting, but the problem is rather convoluted, but to place it in simple terms - The programme is displaying a negative float path AFTER, the zero project critical path, such that the project end date displays a zero total float, whereas within 8 activities further up the backward pass , it is superceded by a -4 total float path. The client intitially created this with ’ladder’ logic and as the updates were non-sequential, I am scheduling to ’progress over-ride’. Is the fact that a) - the ladder logic is constraining the activties (SS +15, FF +15) such that the already critical activity is now beyond a sensible arithmetical calculation, seeing as it began 4 months ahead of sequence (note the SS & FF on said activity)? b) - there is an error with the software ( i doubt this as I have ran a diagnostic)? c) - could the ’Purge Deletes’ function have an impact, as when I re-schedule the -4 becomes - 9 weeks? ..............simply put, it doesn’t make arithmetical sense for this calculation to exist, and the software is having a nightmare of a time trying to cope with it all. I did take out all ladder activities, which resolved the total float path, BUT it still irks me that the problem occured and it also irks that we have still to resolve why this occured. All advice and thoughts will be met with joy on my part. Thanks, Robert