Hyundai 99 - Shakes and loses power upon starting

Hi!

I own a 1999 hyundai elantra with 85,000 miles on it.

My car, upon starting, begins to shake violently and then loses power, I mean refusing to move forward. I have to then put the gear into park and then turn the car off and then on again, and then everything is fine.

Any ideas why this is just starting to happen?

Thanks - any responses appreciated![/b]