Hesitation and Stutter when engine is under load

2.3L 4cyl, 5 Speed Manual, 2WD, 240,000 Miles

My question is:
What could be causing the hesitation/stumbling issue I am having?

High freeway mileage, lifetime of consistent preventive maintenance and inspections.

Past history possibly related to current condition:
MAF Sensor was replaced about 30,000 miles ago. Prior to that change, vehicle would start, but not stay running. When it did run, it was extremely rough, intermittent, continually 'stumbled/hesitated' and would not stay running unless I depressed the accelerator. MAF Sensor replacement completely fixed the problem.

Current problem:
10 days ago (150-200 miles) started to notice the engine running rough. Minor hesitation when accelerator depressed (more than usual). Continued driving. Problem got worse. However, the main problem became the severe hesitation and stumbling as I put the vehicle into gear from a dead stop, pressed the accelerator and released the clutch. Vehicle would noticeably stutter/hesitate until RPM picked up and speed increased. Majority of symptoms/problems isolated to when engine is under 'load' (when starting to move from a dead stop).
To compensate, engine RPM revved to 2000-2500 RPMs before engaging clutch in first gear. High RPM reduced the stumbling/hesitation. Problem seems to lessen when vehicle has warmed up. But it is still noticeable.
Once engine cools, the problem repeats.
Over the past 10 days, the severity of the symptoms has grown worse.

4 cyl. Engine with 8 spark plugs.
Did a cursory inspection of plugs/wires/connections. Nothing obvious. Inspected MAF sensor to ensure plug was securely fastened and air way was not obstructed. Air filter is new. Disconnected battery (hoping that removing power might 'reset' a faulty computer). Symptoms persist.

Does not appear to be idle speed control motor and would think, all things being equal, the MAF Sensor is still 100% functional.
Not sure of the exact time frame of spark plug, wire and fuel filter replacement nor fuel injection cleaning service. Have not put truck on computer to reveal error code, Check Engine light is NOT illuminated.
(However, this may have been permanently disabled during previous routine maintenance, smog inspection and clutch replacement by less-than-trustworthy (and never used again) mechanic).

I cannot recall a specific event or set of circumstances when problem first began.

Vehicle is driven 10-20 miles a day (for past 3 years). 75% of this driving is on the freeway.

Typically use 87 octane gas. Periodically I run 93 octane and/or use bottles of STP and Chevron Fuel System cleaner etc.

Any help/ideas/suggestions/theories/hunches/guesses would be very much appreciated and very helpful during my troubleshooting. I'm a competent (albeit basic) auto mechanic. Not afraid to get in and try just about anything. I do own the Haynes Manual for my truck.

Thanks for your valuable time and insight.
Tom