My car had just been filled with gas then right after I got on the interstate and whlie going about 60 mph, holding the accelerator at that speed, the car stated to decelerate on its on then it would go back to 60. When I got off and was going up ramp at about 15 mph the car started bulking violently. What could cause this to happen?
What year, how many miles?
A failing fuel pump could account for this, but there are other possibilities as well, including but ont limited to a clogged cat converter.
Is it a '96 or later? Have you had it checked for stored fault codes?
We need something to work with here.
When was the last time the fuel filter was replaced?
It does sound like the fuel system. I concur with the other above. One other possibility since it happened right after a fill-up is a problem with the fuel tank venting system. A vacuum may be forming at the top of the tank, preventing the fuel pump from delivering fuel to the engine in adequate quanties.
If the check engine light is/was on, please post the codes.
Does the engine start reliably, every time, no extended cranking?