Personally, I wouldn't have used the term "contradicting". There are other words that would be more appropriate, such as "irresolute" or "indecisive".
Dave has been very irresolute over making the decision if he is going to join the party of not.
I would classify him as an indecisive character - a "ditherer" - always "blowing hot and cold"
See: dithering - definition of dithering by the Free Online Dictionary, Thesaurus and Encyclopedia.
dith·er (dr)n. A state of indecisive agitation.
intr.v. dith·ered, dith·er·ing, dith·ers To be nervously irresolute in acting or doing.
Hope this helps