Car still shuttering/accelerating poorly/perhaps misfiring? Need more ideas!

I've posted about this before and have been given some ideas but so far my car is still acting strange.

I'll describe whats happening:
The engine INTERMITTENTLY becomes incredibly bogged down. I'll press on the accelerator and the engine makes almost no power and begins to shutter as if the clutch was messed up (I've been told its not likely the clutch due to the intermittent nature of it). During this time the exhaust also begins to sound like a diesel (I have flowmasters but they definitely do not make that sound normally). At about 2000 RPMs the engine seems to go back to normal...until the next gear. This problem happens in every gear and happens very randomly. It will sometimes happen on my drive somewhere, but not on the way back. The engine will sometimes act fine, then transfer into this problematic performance. There has been no noticeable specific variables which make the engine behave this way: it can happen on a cool morning, hot afternoon, highway driving, city driving, short drives, long drives...doesn't matter. It seems very random.

I had idling problems in the past which brought up a code about the drivers side hot O2 sensor. I replaced that O2 sensor and the idling problems went away. Now this is happening. During the time that the poor idling was going on, I checked/replaced many things that I was told could be the culprit. This is the list so you know what I've done so far and what should be eliminated from the possible culprits for this new problem:

-Replaced driver side hot O2 sensor
-Check all vacuum lines (that I know of) for any leaks, etc.
-Checked EGR valve functionality/tightened fittings connected to EGR
-Replaced all spark plugs
-Cleaned MAF sensor
-Replaced fuel filter

I'm going to replace the passenger side hot O2 sensor today to see if anything changes since the car has 117k miles and it wouldn't hurt to replace that one since I've replaced the drivers side a few months ago. The car is not throwing any codes on the dashboard and I don't have an ODBII scanner to check of any "pending" codes. If anybody has any ideas on what else I could check out, I'm all ears.

Thank you all

bigkeeko said:

Have you had a scanner on it since? Any codes?

Fuel line blockage? Injector problem? Has it been re-mapped/re tuned or had parameters changed?

Have you ran an injector cleaner? or had any of the fuel supply parts changed? Fuel pump ok?

Just trying to get an idea here.

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I have not gotten a chance to put a scanner on it since the initial idling problem.

Car has not been tuned. What is the best way for checking fuel line blockage/faults at the injectors?

I don't remember putting fuel injector cleaner in it - if so it would have been a long while ago. No fuel supply parts changed. Not sure about fuel pump - how would I check that?

Thanks!

bump

Well, as I predicted, replacing the O2 sensor did not fix my problem.

Could anyone give me advice on how to check my fuel delivery system in order to see if that is the cause of this problem?

trinity_gt said:

Check the fuel pressure at the rail, preferably as you drive and experience the bogging.

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What do I use to check the fuel pressure?

I'm borrowing an OBDII scanner that monitors real time data. However, it doesn't read off the absolute fuel rail pressure for some reason (in the manual it says its one of the things the scanner reads - could be related to the scanner not recognizing the vehicle). It does show long term and short term fuel trim...is that of any relevance?

Edit: spoke too soon