Without knowing the brand and model of your primary filter, it is difficult to comment of the level of vacuum you are experiencing, however a quick look at the vacuum gauge which is available from Racor
Filters for their turbine series might give you a base line for further action. The Racor
gauge gives from 0 to 7" of mercury
as an acceptable level of vacuum, from 7" to 10" of vacuum as being in the yellow segment of the gauge, and over 10" of vacuum as being in the unacceptable "red" section of the gauge.
Looking at the Racor information for their spin-on filters, the typical maximum new filter vacuum is about 1" hg.
All this points to your vacuum level as being rather high, but with a wildly oscillating gauge needle it is really impossible to tell what you have. What you might try is to install a restrictor between the gauge and the vacuum source to dampen the pulses. A fitting with a drilled opening of about .015" or .020" might work, or you could install a fluid filled gauge as Racor uses.
Hope this gives you something to think about.