Fluctuating Vacuum Gauge Issue: Need Help

So I heard a lot of engine issues can be diagnosed by reading the vacuum gauge, but I have no idea what my issue is.

When at cruising speeds my vacuum gauge would jump from around 15 inches of mercury to 20 inches of mercury. I would feel a sudden decrease of speed while this occurs. However if i increase throttle it would go away. this only happens in 5th and 6th gear and during constant speed. At idle, the vacuum gauge is fine, the needle doesnt jump. Any help is appreciated.

The car has bolts ons; intake, headers, and tune. Also, i dont know if its a coincidence but this issue started when I got aftermarket resonators for the my exhaust.

Gotta be a tune thing.

Thanks for the reply. I've had the tune for about 6 months with no issues until this. Maybe I will give the tuner an email and see if its a known problem.

Well your gauge should move a bit i believe. From my understanding if your load varies, it will demand more boost, or if your load decreases the opposite will occur. It's not an On/Off switch, there are many different circumstances that could cause this which are normal. I'm not sure what the negative reaction here that's being asked about as the ECU will demand different pressures for different things.

You say there was a felt loss in speed...but was there ACTUALLY a loss in speed? Should be pretty easy to tell lol. Are you able to do any logging?

Spykexx, I had a friend ride with me and he also felt that sudden "jerking" motion when the vacuum gauge jumps ( about 5 inches of mercury). The action is quite quick though so when i said loss in speed it was just a way to describe it.
This will happen with my foot lightly on the throttle maintaining speed and also if i put it on cruise control at low rpm. It almost seems like the throttle is closing and opening again.

As for the logging... Stupid me NEVER thought about it. Excellent idea. I will do that today. Thanks

Ahhh ok. That's what I meant when I said I wasn't sure what the actual symptom/problem was you were trying to overcome other than loss of speed and drop in pressure.

Yea my go to is logs. Can tell you a lot about what is going on! I actually keep my SCT x4 hooked up at all times. Check out your load, temps, knock sensor, boost etc. Should point you directly to what everything ELSE is doing when you get the power dips.

Try cruise control see what it does.

Nevermind you did. Weird. Maybe faulty throttle body.

AC compressor is likely kicking on. The change in load will cause a change in vacuum pressure. There could be a problem with it though if it is pulling more than just a barely noticeable amount of power. Maybe make sure something isn't going on with the compressor or the PCM/tune.

Wow I totally derped on this one. Was thinking you had an EB haha.

AC is kicking on.

I realize this is a very very dead thread, but was there ever any resolution to this? I believe I may have the same symptoms on a 2020 6spd GT, see my thread here:
https://www.mustang6g.com/forums/threads/surging-throttle.176788/#post-3573318