You don't really need any VBA for what you're doing - simply do a wildcard Find, where:
Find = [.\?\!^l^13]*[.\?\!^l^13]
As per VBA, of course, this will identify anything with a period in it as the end of a sentence but, other than that, each time you press 'Find Next' in the F/R dialogue, it will select the next 'sentence'. Since you're wanting to stop to add formatting, there's probably not much benefit in having a macro.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]