At times black smoke from the exhaust pipe. runs rough. then dissappears and runs smoothly. sometimes runs with hiccups.also disappears for a long time.

worn out piston rings require replacement..

If you have not replaced plugs start there A bad map sensor will also cause black smoke and poor running

Possible blown head gasket......

Possible vacuum leak and or internal engine issue, have a cyl compression check done along with cyl leakage test

signs of a bad fuel pressure regulator-- the vehicle will run rough, the engine will not start ( usually in the mornings,) many times engine starts but stalls shortly after starting. a bad regulator can result in fouled plugs. replacement is only cure. 1- black soot on plugs-2-black smoke from tailpipe-3- running rough-4- stalling, good-day! repost if need be.

Please post other info. model, mileage, engine size, year, any work done to it recently?

What is the 8th digit of the vin?

Check the fuel pressure regulator, it may have gone bad and running the engine rich. Good luck

you don't say what year, if it has a carburetor it's flooding (Needs a new float) Way too rich

remember this
Blue smoke OIL
black smoke GAS

Take it to an Autozone or O'Reillys and they will test the computer for you. It should have set a code, but you don't know if the code light works so its your best bet. Sounds like it is running rich and the crankshaft module is not advancing the timing.