I had a problem like this on my Dodge Durango. After sitting during the week, I'd take it out for errands. I'd park and go into a store. When I came back out, I'd start it and wouldn't be able to move out of park.
After 20 minutes or so, it'd suddenly be able to move out of park and I'd go on my way. It wouldn't do it again all weekend.
The next time I'd take it out, usually 1-3 weeks later, it would do the same thing.
When I took it to the shop, they were perplexed. Finally one mechanic said it was the gear shift cable. He said it would get stuck in the steering column so the gear shift couldn't move. He moved it inside the column and I haven't had a problem since.