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.