Intermittant and consistant running like crap can also be caused by a so-called 'crank cancer', as the timing is affected (the crank sprocket triggers the crank sensor, which all the electronic timing events are based on) as well as valve events.
My personal opinion is that people don't tighten the crank pulley bolt properly when R&R'ing the crank pulley (or perhaps it's not tightened properly at the factory, but I'm sure there aren't that many factory tightened crank bolts left ...). This reduces the clamping force on the timing belt sprocket, and the non-uniform loading on the poorly clamped crank sprocket eventually starts it rocking on the key, which leads to crank sprocket and key wear, and if left long enough, the crank gets damaged.
The sad thing is that someone who performs regular maintenance is more likely to be R&R'ing the crank pulley, and since it's such a PITA to tighten that bolt properly (you really need a flywheel holder), it's not tightened properly and gets loose, causing sprocket/key and even crank damage. Of course all items are obsolete ...
To check if that's the issue, remove the upper timing belt cover, and using a wrench, slowly rotate the crank forward and backwards. The timing belt MUST move with the crank. If there's a significant pause in timing belt motion response on change of crank rotation direction (or heaven forbid, the bolt is loose), then the crank sprocket is rocking on a damaged key, and the sprocket and even crank may be damaged. You can also check if you can rotate the cams without rotating the crank. If you catch it early enough, the crank may not be damaged.
Another cause for low RPM hesitation is a bad MAF. I've seen this countless times, even on the same car with a MAF that's already been replaced. It seems to affect the late model cars more with the F1ZZ (IIRC) MAF. If the car runs better with the MAF disconnected (ignore poor throttle response), the the MAF is likely the culprit. Swapping another bad MAF from another car may not help diagnose that one.
Finally, even if it's only to borrow one, a TwEECer RT will allow you to SEE what the EEC is seeing for sensor inputs, like ACT, TPS and ECT etc. and what it is commanding for timing and fueling etc. Even if the sensor is good, bad wiring may mess up the signal. Instead of spending money and time replacing sensors 'just in case', money spent on a TwEECer RT can help you spot the problem, and then you have a tuning tool forever. A sticking throttle cable may prevent the throttle from opening all the way, and you can spot this by seeing a WOT TPS voltage that is low. KAM trims that are pegged at 10:1 and HEGO's that are pegged at low voltage during closed loop can tell you that there isn't enough fuel pressure/or dirty injectors. You can SEE if your HEGO's are working properly, or are just sluggish. The list goes on.
Tighten your crank pulley bolts properly people!
P.S. to address the thread title, our FPR will only help a car that has an over-sized (190 or higher) fuel pump, unless the stock FPR is actually bad (rare). A full discussion of fuel pressure and how to measure it is here:
http://www.shonutperformance.com/FPR.htm
