06 Vibe base Bucking / hesitation problem

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I bought an 06 vibe with a 5 speed a few months ago and ever since, when accelerating it's very difficult to shift smoothly at higher RPMs because the engine seems to start hesitating, almost like I'm pushing the gas pedal up and down. It does it consistently in maybe 1 or 2 second itervals. It's not the clutch slipping, the engine doesn't rev away, and it happens regardless of what gear I'm in. It happens roughly at 4k rpm and up. The cars perfectly drivable otherwise and shifts fine. However it's bad enough that I just try to completely avoid that rpm range. I checked engine codes the only one is the typical p0420... No misfire codes or anything else. I'd really like to figure this out because I love the car but lately I've been driving my other vehicles more due to this issue becoming annoying. Thanks in advance for any help, suggestions or otherwise I really appreciate it. Also if it matters, the car probably has very close to if not more than 400,000 miles, and otherwise runs fantastic.