Engine misfire under vacuum

[GCMX5]

Hey all.
I am hoping to get some guidance on what else to look for with this.

The car;
This is in Morticia, my NA8. No significant engine mods other than a replacement air intake box.

When it started;
The car sat idle for a period of time and began when I went to drive it for the first time after a long period.

What it does;
At start-up, idle and under light throttle,engine seems fine and smooth.
The engine seems to misfire (like it's hitting a rev limiter) as vacuum levels increase. The misfire can happen when stationary or when driving.
When driving, if you ease the power in as revs increase,you can get full revs from each gear.
At low speed or stationary, it will misfire at anything more than about 20% pedal application.

What I have checked;
At first I thought it was bad fuel so I filled it with fresh 98 and a bottle of injector clean. It has had 2 full fresh tanks of 98 through it and is still misbehaving.
I replaced the fuel filter to no change.
I cleaned the throttle body with carb cleaner, no joy.
I have pulled the plugs and all are a delightful light and dry colour with no build-up of any description.
Whilst I am yet to replace any of the vacuum lines, an initial inspection doesn't show any obviously perished offenders.

My next line of thinking is to pull the injectors and clean/replace and/or check how clean the pump is in the tank. Given that both tasks take moderate effort, I thought I'd put it out there to see what else I have missed thinking of to check.

Thanx in advance.

[Roadrunner]

Replace your ignition leads or swap with someone to test before you go pulling injectors and pumps.

[PaulF]

My money is also on ignition leads, or (less likely) a coilpack on its way out.

[bruce]

Spark plug leads!

[StillIC]

Whatever the problem, it is likely ignition system based. Symptoms not dissimilar to failing igniter/coil or leads as the people above suggest. And check that your leads are clean, fit to coils and plugs are good (not loose connections) and not touching each other or too close to each other, although this shouldn't matter with modern, new parts.

If good check for cracked spark plugs, just in case, but this is normally associated with misfire under load. I once had this problem that took years to find as the spark plugs were brand new when the issue started and I didn't consider that they'd be the cause.

If they are all good then I'd considering checking cam angle sensor to ensure the ECU is receiving the correct firing signal. This is needs an oscilloscope or a part swap method if you have access to a known good one.

[GCMX5]

Thank you to all suggestions.
Leads are only about 12months old and I put some good 8mm leads in. I wouldn't expect they would be crook but anything is possible.

For shits & giggles I thought I'd change the plugs and see what happens. At the same time, I checked all the leads were pushed in well on the coils and plugs.
I had the leads out of Mindy ready to swap if needed.
Well I'll be blown over, you guys were all correct. With the new plugs, about 95% of the issue has been resolved. With any luck, driving it may clean up the rest.
Thank you for the continued assistance of this community.
As a new local in the Newie area, I'll look forward to making new friends and connections with you guys in this area