1999 Hyundai Elantra Problem Car. Period.

Okay, for starters, hello!
I hope I get some help here.
I have a 1999 elantra,
Automatic car.
I just replaced my cracked radiator,
In the process my water neck broke (thermostat housing)
So that was replaced as well,
It all came out fine, I drove home,
While driving home it started to snatch so to speak. Like a strong surge that would almost stop me completly, now it made it home, I park it, go to sleep wake up, go to leave for work, it starts just fine, stays running. But when I put it in R it starts, then acts as if I put it in N. It wont go anywhere.
I can do the same with putting it in Drive,
I've heard 'transmission transmission' but if that was the case. I wouldnt have went home.
Could you help me? Is it something loose?