Ok...so having had this issue and more with my MPI...hopefully I can pass on some useful info.
Firstly...don't mess about with normal plugs...these need Irridium plugs..pure and simple. Either the NGK you mention or the AC delco superseded 993 model.
Above said, I'm not thinking this is the problem. Given your symptoms, it most likely lies with the throttle position sensor.
You also need to replace the rotor as well as the cap. Pointless doing just the cap, for the sake of it and given this is a well known weak point.
Failing the TPS...IAC is a less likely culprit (but another known weak point) and will usually show up on faults.
Also most likely not to blame would be the SW pressure sensor....but this will put the MPI into a 90% or less gaurdian mode and make things play up.
I really do think this may be a TPS issue, also worth thinking about is a problem I discovered by messing around.
These engines take a fair bit of electrical power when running...if your battery isn't up to much it can be an issue. Even when the alternator is pumping out 14v dc, a failed battery cell can loose this to void and not give the required power to the engine.
Unlikely your problem, because this issue I found was causing the engine not to fire up....and not in the usual way.
To explain...the engine cranked over perfectly, making me think the battery was good...and would only try to fire when I released the key.
Turns out that the battery had enough power in it to turn her over quick enough, but the cranking amps required tool just enough power away from the coil(really high demand) to cause her not to fire up.
This was proven by cranking and when releasing the key (just when she tried to fire upon release) quickly turning key back on...she fired straight away because there was no longer such great ampage taken away from coil.
This also suggests a weak coil. I replaced battery and haven't had the issue since.
Anyway, I digress. As mentioned, likely a TPS called for, along with iridium plugs and a rotor.
Let me know how you get on.
PS....all the above is assuming that you aren't picking up crap from the fuel tank pipe and first line of defence metal gauze on the pick up isn't blocked !