2002 Toyota 4X4 my trucks RPM's have not been consistent

2002 Toyota 4x4 truck mileage: 79,000. For the past two-three months my trucks rpm's have not been consistent. It fluctuated even when I did not accelerate the engine. In the past two weeks, it has been getting worse. The climax was a few days ago when I got onto the freeway and experienced a craze with the rpm’s as I changed gears. Ultimately, I was stranded on the side of the freeway. I changed the clutch as my mechanic advised and it has gained a kick when I shift gears. The rpm's still fluctuate slightly on neutral, first or second gears. Once I get going it is not a problem. Obviously, the clutch was not the source of that problem. I am assuming it may be the fuel pump or filter. What do you think the problem could be?