Starts but loses power then cuts out; at first would run 20-30 min before losing power; is not overheating; replaced fuel pump and filter; no sign of water/coolant in oil, no bubbles in coolant, no moisture coming out of exhaust; tried removing fuel cap to make sure it was venting; tried removing air hose at intake manifold to be sure air intake adequate; get black smoke at start up, but not after, this was typical before started running poorly; tried seafoam and diesel 911;