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Check the health of the knock sensor. It is an inexpensive part that "listens" to the engine, senses knock and automatically retards the timing. They get full of crud etc. and give false information. Incorrectly retarded timing will give you a noticeable hesitation.
A "slightly" faulty AMM, i.e. during low air mass conditions, can output a too low "air mass" signal to the ECU causing a too narrow (lean) basic injector ON pulse width. Carefully check the AMM's wiring harness plug. Slight misalignment of the female contacts can cause ECU input problems.
Clean throttle body.
Is your temperature gauge correct? Is the engine heating up properly? Is the thermostat working properly?
The fuel pressure regulator is worth a careful inspection. The fuel pressure must rise instantly in response to the vacuum signal fall that accompanies a throttle opening. A hardened diaphragm might be causing the fuel pressure that has been lessened by the fuel pressure regulator to not increase as rapidly as it must and you won't get the appropriate fuel quantity in spite of lengthened injector duration. Try an acceleration test with the vacuum hose pulled off fuel pressure regulator. Easiest way to check the FPR is to pull the vacuum hose off of it while the engine is idling. If the idle picks up, your FPR is good.
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