If you are on PS3, there is a 12 step process to trying to solve this.
Take-Two suggests PS3 GTAIV fix - PlayStation 3 News at GameSpot
The latest solution is to simply not log into the PSN.
I don't know if any of it works, luckily my only freeze up was solved by restarting the console.
Sorry to hear about your problems. R* said on Wednesday a patch to solve all the current problems could be hours away or days away. I had a lot of respect for R* when I played the game, I lost it all when I saw that comment.
EDIT - Didn't see Allstar's post about the PSN, it is confirmed then.