i had some what siilar problem on a trip to colorado in my 67 camaro 396 its a new carb and all and up here in sd ittl lite the tires throuh 3rd gear from dead stop in drive ..its an auto ..for now lol and when i got to colorado it would sputter and almost die on hard aceleration then it would take off and bam all the power in the world...i belive my problem was my carb is jetted for south dakota ellevation and the increase in height in co and less air led to my problems cuz when i got back to sd it runs like a champ again ...not sure if this helps at all ohh and ps my timing was dead on were it should be well good luck