Loosen up the exhaust pipe ahead of the muffler. The symptoms you're describing are classic for an exhaust system restriction such as a clogged muffler or catalytic converter if you have one. Another diagnostic procedure involves connecting a vacuum gauge to a source of straight manifold vacuum such as the brake booster line or the transmission modulator and hold the gauge down with a windshield wiper so you can watch it as you drive. When the bogging-down symptoms begin, the vacuum will drop to nearly zero and stay there anytime you're trying to accelerate. Loosen an exhaust pipe flange somewhere ahead of the muffler and see if the performance improves.
Jerry

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