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Not a knowledgeable mechanic at all, so apologies for any idiocy below. But greatly appreciate any help you can give so I can cross-check what my garage and tuner may tell me.

2007 WRX w/114k miles. It was bone stock until a few weeks ago, when I found a tuner to disable my secondary air pump codes. (Valve stuck closed). We did not block off valve physically, it's been stuck closed for like 2 years. I asked what would happen if valve suddenly got unstuck, he said not a big deal, performance-wise, but I'd probably hear louder exhaust and could then take it in to have the valve blocked off. In other words, no harm in just waiting to see if that happend and then fix. Car was totally fine after he disabled the codes (and it was blissful to have cruise control back). :) That was abt 3 weeks ago.

Last week on a below-zero morning, when accelerating to enter a highway, car lurched and sputtered as if out of gas. CEL began flashing, and then went to steady-on. From then on, it's done that (sputters and flashing CEL) anytime pushing the RPMs beyond say 3500. I can kinda mush it through its gears to get up to full speed, but it has no ooomph.
So, my shop just called, they pulled code P0303 = third cylinder misfiring. They're going to check spark plugs and do a compression test. He said he didn't see any signs of head gasket issue.
Any questions come to mind that I should be asking them / pushing back on?

Some other info, aside from the recent tune, in case relevent:
--Temps were well below zero that morning. I'd driven about 6 miles before it happened, but car was probably still pretty cold. That said, I didn't have it floored, was just moving through the usual range of 3rd gear.
--I had just filled my tank the night before. Drove maybe 12 miles or so on that new tank before this happened. Tank was about half full when I filled up (I'm careful not to let it get too empty when it's cold like this). Nonetheless I've put two bottles of HEET in, in case maybe it was water in the gas? No improvement.

What do you think's the probability that this is related to the recent tune? I'm not quick to blame-- this is an older car and not surprising for a major repair to crop up. But I can't help but wonder if I brought this on by getting those damn codes cleared.

Sidebar: I was really hoping to keep this car on the road until at least 200k, so if compression tests ok, then we're thinking replace ignition coils (as long as they're pulled out) and timing belt. (Need to think about it--kinda tough to spend $2k on an older car (but I've had a few years of no car payments, so as long as the fix is durable, I'm coming out ahead).
 

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Could be bad fuel, maf issue, vacuum leak, exhaust leak, bad plugs, bad coil, tune issue, compression issue, I've seen some threads mention lightened fluwheels, a number of things.

It could likely be the tune, I would wait and see what the shop tells you.

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