Running rough after head installation

Hi! I have a 1.8t Volkswagen Jetta 2002. I just took the head off of the engine and replaced the valve stem seals. I installed the head and started it up and it ran just fine for a few minutes and then just randomly shut off. I started it back up again and it is running rough now. I am out of ideas. I’ve re checked and double rechecked the timing and considering it ran completely fine for a solid amount of time I wouldn’t think that it could be the timing but I’m not for sure. Any help is much appreciated!! Thanks