Stock '01 S40 152,000 miles.

I've been getting a P0171 code for about a month now. Of course it only started after I gave the car a 150K mile tune up.

The car feels normal at 1/2 throttle. Idle varies about 10-20 rpm according to my ScanGauge II. 3/4 and Wide open (WOT) though, it misses. Still has some balls, but not steady - it lurches. This is my first S40, so I'm not sure what to expect performance wise, but I know something is wrong. The only code has been P0171 and I cannot clear it with my ScanGauge II. Even for a second.

Here's what I've done so far;
New plugs - NGK BKR6ES @ 0.30" (P0171 showed up before this plug change);
New air filter;
Cleaned the MAF;
Cleaned the throttle body;
Changed the second catalytic + flex pipe (flex pipe was cracked);
New alternator;
New serpentine belt tensioner;
New fuel filter;
Checked for vacuum leaks visually, with carb cleaner and propane;
Checked for injector seal leaks with carb. cleaner;
Checked fuel pressure -
35 psi at idle;
31-32 psi when coasting;
43 psi @ steady 30 mph going up a steep hill;
50 psi @ WOT.
I think these figures are within spec.
Plugs have been in there for about a thousand miles or so and look normal - slight tanish-whitish-browinsh. Definitely not black or coated like running rich.

I'm suspecting the MAF, but don't have $250 to spend on just a hunch. I guess you can test a MAF if you know the fuel trim levels. I think the ScanGauge II will read the fuel trim, but I'm not sure how to get to that menu and the instructions that came with it are no help.
Does anyone have a how-to on testing the MAF? I don't have another S40 to swap with.

This car is killin' me with the CEL and the miss and I need help. I've been avoiding throwing parts at it because I have been trying to diagnose the problem and don't have the cash to just "try things".

Little aggravated.

:edited for spelling.