2003 Aztek Acceleration Problems above 3500 RPM

[AztekLver]

My Aztek started running roughly on the freeway the other day. I thought that I possibly got a bad tank of gas. Added STP fuel treatment. While parked and running I accelerated slowly with no problems until I reached 3500 RPM and above. Aztek started bogging down and could not hold a constant RPM. Decreasing speed/gas pedal pressure to maintain RPM below this point was smooth. The check engine light did not appear at any time. Not sure where to start in the troubleshooting process and mechanics who know Aztek's are few and far between.
Has anybody else had this problem and what was the resolution? Any help or guidance would be appreciated!

 

[melsg5]

You cannt increase the engine rpm beyond a certain limit when not in gear.
When is the last time basic maintenance was performed, plugs, air filter, fuel filter, etc.?

 

[AztekLver]

Maintenance was completed this summer (June 2019). Was running great until a few weeks ago when I was driving 60 - 70 mph. Seemed like I had to almost push the gas pedal to the floor to go those speeds, and even with constant pressure, the speed and rpm's were fluctuating erratically. I have only driven it about 250 miles in the last year, but it passed emission tests with flying colors and was running great 2 months ago.

 

[melsg5]

If you are sure the fuel filter was changed in June, recommend having a fuel pressure test to see if the pump has an issue.

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[AztekLver]

Thanks. I appreciate your response. I'll start there.