Stumbling under boost and P303

I've been reading up on various old threads about P30x codes. It seems like it can range from air leak to burnt valves, with most people throwing parts at it until it's fixed.

My problem is that the car seems to idle ok (little rough), runs fine under vacuum, but when boost hits it stumbles and the CEL flashes. The only code is P303. Plugs are passed due (114k on them) One other possible factor is that I put some regain fuel cleaner in the tank at the last fill up (1/2 tank now)

I'm thinking that the cleaner either dislodged some crap that partially clogged the #3 injector, or it dislodged some crap and fouled the #3 plug.

Other than changing plugs, and maybe swapping out the #1 and #3 injectors, what else should I do to diagnose?

UPDATE: I think I found at least part of the problem

I first swapped the #3 and #1 fuel injectors, cleared codes and went for a drive. Still the same feeling, and the code is the same (P303)

So I started to work on changing the plugs with the intent to swap the #3 and #1 coils at the same time. When I went to unplug the #3 coil the connector fell apart in my hands. After a few moments of panic I found out that this is pretty common and there are replacements available online. I have a feeling that the connector was cracked before and therefore not making a full contact (part of the connector stayed in the coil). The plugs look pretty bad too so crossing fingers that the connector and the new plugs fixes it. I guess I have to wait a couple days for the part to arrive so I can see.

Side note: F-you Subaru for making this so tight and a giant PITA (and literal pain in the hands)

I had to replace both the coil connector and the injector connector on #3.

It’s tight getting in there, but I found that if I pull the intake box (1 bolt and 1 nut) it’s pretty open to get at that side of the engine.