I'm a Provisionist and an ex-Calvinist. I had some doubts about Calvinism for awhile, particularly with regards to the problem of evil and limited atonement.
While I am sure Calvinists don't actually teach that God is the author of evil, I personally couldn't reconcile it with my own understanding of Calvinism, and saying that God is the author of evil just seems the logical conclusion of Calvinsim. After all, if God preordains everything, then does that mean he preordained evil?
I was a four point Calvinist because I thought it was obvious from Scriptures that God died for the sins of the whole world and does not with to see anyone damned for eternity.
What really started to dissuade me from Calvinism was the idea of corporate election. I used to believe in unconditional election, because I thought the alternative meant that God chose people based on foreseen faith, and therefore makes God dependent on man's will.
Corporate election, on the other hand, means that God has elected a people to be his. While the Church is made up of individuals, election is not understood as being primarily of individuals, but as part of being a member of the Church. Christ is the elect individual, and all who are in him become a part of the elect. It's supposedly closer to the 1st Century Jewish view than Calvinism is in that the Jewish people weren't as individualistic as modern western Christians.
Once I changed my mind about election, the rest of Calvinism unraveled for me.