There is a limit with credits (and many others) in 2003.
As a rule of thumb assume that if you have more than a screens worth of data off slide - expect trouble!

One solution is not to use credits

Position the text off the TOP of the screen and apply Fly In

Set to from BOTTOM
Text Animation = as one object
Manually type in a new speed of say 25 seconds in timing. (you can't choose this from the options)

See if that works

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