1997 Chevy Truck engine trouble

I just recently replaced a head gasket on a 97 chev z71 witha v8 vortec and every once and a while it bogs out really bad like its missing or the timing is out, but sometimes it runs fine. I thought it might be the risers or timing chain or something like that that I put togather wrong but I am not sure and I dont really want to take it in, what do you guys think?