Engine Model : 4JG2 Bighorn (1995)

Car has been running fine until yesterday, It revs-up by itself up to 2000rpm then down to 1000rpm then goes up 2000rpm...

It emits heavy black smoke while revving up, both idle and running.

what maybe the cause of this problem Pls. Help.

I want to get an idea before have it checked by a mechanic.

Thanks