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What year car? Are there any CELs or codes?

Both valves leaking quite a bit. Head has to come off. I would suspect valve springs as well given the circumstances of the failure

Late 2013, just right outside the recall range.

CEL was blinking intermittently when it happened.

P0304 showed up (for me), not sure if any other codes popped up at the workshop.

No audible rattles or other strange noises? A broken or week valve spring could indeed cause a misfire.

None out of the ordinary.

Car started spluttering in stop-go traffic with no warning beforehand.

Sorry for all the questions! Stock? no mods?
Could be a weak spring but it would be very unusual when outside the recall window.

No need for apologies! Happy to supply answers.

Cooling mods like radiator & oil cooler, suspensions, brake, fluids. Nothing to increase stock engine output.

The only thing that has me really questioning valve spring issues is that they usually show up at higher RPMs but you said it is mostly at lower.

Yeah, that's what they told me too. That's why they didn't believe it would be the springs at the beginning.

When you say it shows up at higher RPM, does that mean that the misfiring should have started only at high RPMs? or just the symptoms (strange noises as you mentioned)

Valve springs generally fail at high rpm when they're under stress but a flawed or overworked spring can fail anytime regardless of rpm.

Ah! Thanks for the clarification. Would make sense since the car has been tracked regularly for the past 5 years.

How many miles on the car? Any chance of carbon buildup causing valves to not seat?