stumble from stop

I have a 98 k1500 5.7L vortec that I just rebuilt and got running. The engine runs fine down the road but under medium to heavy acceleration from a dead stop it has a decent stumble and occasional POP that sounds like it's coming back through the breather. also at Idle in gear the engine surges up and down about 100 rpm's. No codes are being thrown, but could the corolation be off just a little or what???