Ive noticed that sometimes at a red light my 2019 sti would sometimes have the rpm "bounce" which I now understand is seen as normal on these cars. However I more recently noticed that my vacuum at idle fluctates rapidly by about a pound to a pound and half. This only happens sometimes and only when the a/c is off. I also notice it happens when the car is on more of an icline. I did notice this when it got colder outside so could these be a factor? The only mods I have done is a perrin pitch stop mount which I had to take the intercooler off to install but made sure to tighten down correctly. The car has always had an idle vacuum fluctuation of .3 of a pound. I took it to a subaru tech and they said "nothing was noticed". Can anyone advise anything about the fluctuating vacuum?