Hesistation after hard acceleration, sometimes
 

I'm really frustrated with this issues, I can't seem to diagnose what is going wrong. I had an issue where my 98 2.5 wouldn't hold an idle when hot. It was waver up and down and eventually stall. Found the ICV was cracked so I replaced it with a new gasket. The idle problem was fixed.

Now the car is having intermittent issues with stumbling or hesitation. I thought it was old gas, but have run two tanks through since then as well as injector cleaner. It tends to happen right after acceleration at nearly open/wide open throttle above 5500rpm. The car will pull fine, but after I let off, will stumble for a bit. If you short-shift for a bit it will calm down and drive normally. If I drive always under 4000rpm I never see any issues. I can commute fine but am always worried it'll start acting up. The idle is right about 600-700rpm, but is a little unsteady. I don't remember it moving around at all before all these issues.

I cleaned the MAF (looked really clean already), cleaned the TB and ICV passages. I tried to check crankcase vacuum in case it's the AOS, haven't been able to adapt my vac. gauge to the dipstick but if I open the oil filler, the idle rises and gets rough. There is def. some vacuum but I can easily lift the cap on and off.

I check codes (no CEL) and there's nothing stored.

Any ideas? I've got a fuel filter to install, mainly because it's got 133k and might have never been changed.

Do you have/have access to a scan tool that can get your live data? Find out what your fuel trims are at idle, and have a passenger watch then for you under hard accel. See what they do and report back

Couldn't get the acceleration readings, but when hot, at idle:

RPM between 780-815
MAF between 3.8 and 4.3 (I believe g/s, meaning 13.7-15.5 kg/hr)
Short trim 1 between -3.0 and 3.9, mostly -3 to -0.8
long trim 1 is 3.1
short trim 2 between -2.3 and 2.3
long trim 2 is 0.8

No stored or pending codes.

And of course the car runs fine today. Romped on it a little bit in the country and it didn't do it's post-acceleration bogging.

I was going to take some propane and check for vacuum leaks but I doubt that a vac leak would "correct itself."