1995 Pontiac Grand Am loss of power then a plume of black s

I jus bought a 95 grand am and am pleased so far except when I am driving on the highway the car loses power then a plume of black smoke comes out of the exhaust and power is restored..... I am jus curious as to what might cause this..... the guy I bought it from said he thinks it has to do with the timing.... I think it could be a clogged fuel filter.... my father in law thinks its either those two or just old gas.... any info would be appriciated.