hello I have a 95 sable one family owner 49K miles 3.8 v6 runs great but sometimes engine stumbles or misfires and black smoke from tail pipe loss of power going up hills etc could it be fuel filter bad spark plugs ignition coil issue I did have both head gaskets replaced at 48K as it kept eating antifreeze every few days any thoughts out there? no check engine light comes on but is there a way to get codes from the computer or do i need to take it to auto dealer thanks guys