I have a '94 4runner v6 5-speed manual w/ 146k+ miles. When accelerating in 1st and 2nd gear no problems. Even under heavy acceleration. In 3rd, 4th & 5th gear, when accelerating, rpm revs high but the speed stays the same. When I let off the gas and let the rpms drop then accelerate slowly the speed accelerates with the rpms. But if I accelerate hard again the rpms go up but the speed stays the same. No lose of power. Just no gain of power. Any thoughts? 