Oh brother! Stop throwing money at it! PO303 simply indicates a misfire at cyl #3 (rear,
right side). First check its spark plug for a missing anode (famous on the plats), gobs of oil
from a cracked spark plug seal in the valve cover, or loose wire insufficiently pushed onto
the plug.
If all ok, redrape the #3 wire AWAY from exposed metal at least a half inch. If the wire was
clipped in adjacent metalware look for a tiny white spot on the cable, indicating high
voltage shortage to ground (hence the misfire). Replace that wire if compromised. Check
for corrosion where it seats into the coilpack too, as those brass contacts can rot away.
Then test at idle and upon acceleration if there's still stumbling repeat a few times in order
to get a code to set up. If still 303 only you may have a valve problem in the head, which
I'm not sure I'd chase unless this 13yr old is otherwise pristine.
But before giving up change the coilpack and all 4 ignition wires, since you've already
bought them anyway. Again, be careful to NOT dress the new wires too closely to
metalware; those silly plastic "dresser" clips are more problem than not. Floppy wires out
in space are NO problem.
If dealing with the ignition doesn't solve this try switching injectors on the left head. If the
problem travels to the mate then pop in a new injector.
Backing up, the shuddering from misfire is very much correlated with dampness.
Following rainstorms I'd often found a real lightning show in the dark when watching raw
Subies idle in my shop before repairs. You'll also generally hear a "tick" with each 1/4" or
so arc of spark to ground. That may be the case if a coilpack tower is cracked, too, but
usually they short internally with old age. I like NGK silicone wires, but they're often
cloned. A bottle of strong fuel detergent isn't a bad idea, too, as well NEVER letting fuel
level getting below 1/4 tank hereon. Good luck. These are usually resolved easily, unless
you've a bad valve in the head.