help understanding vacuum readings

I have been doing some testing to track down a problem with my truck and the vacuum test's have me a bit stumped.
I could really use an opinion from those who may have more experience in interpreting results from vacuum test readings.
the truck is a 91 2500 over 8600 gvw 5.7 4l80e.
I took my readings by putting a tee on the MAP vac line, it looks like that is the only true "manifold vacuum" available.
I get a solid 19 HG, (whatever "HG" is) though the gauger starts off at 16 HG and climbs to 19 over a short period while the idle settles. before the engine warms up to temp (just a few minutes since it is pretty close to that temp) I get a very intermittent 1 HG drop, I can hear a slight miss in the engine that goes along with the drop as well, after full temp is reached this gets a little more frequent and if I put it in gear with the park brake set it gets very noticeable and goes to an intermittent 3 to 4 HG drop with a very rough idle, there is no rhythm as in a bad cylinder or two it just runs bad.
The reading that really has me is the check for exhaust restrictions, the manual that came with the pump/gauger said to bring the RPM'S up to 2500 and if there is a drop in vacuum while pushing the accelerator there is a restriction or over active EGR problem, well while giving the truck gas no matter slow or fast there is a drop in vacuum but it comes right back up in a snap once I quit pressing the peddle or back off slightly, it will even go over 20 HG for a few seconds after this.
The manual that came with the pump/gauger is not very specific in some areas and has a few misspelled or omitted words in others so I'm wondering how to inter pet these readings.
My Haynes book does not even cover test's like this so I'm not sure where to go next with this.
I have tested the EGR as outlined in my Haynes book the EGR and it's solenoid both tested out as good though my pump/gauger explains a different set of tests that I have not yet had a chance to perform on the EGR valve itself.
I have not noticed any change in tone or flow from my exhaust, I installed a new high flow catgo cat and headers 2 yrs ago, though the muffler is a fairly old 3" flowmax unit like I mentioned I haven't noticed anything to lead me to consider a restriction.
A bit of help in understanding my results from the vacuum test's would be greatly appreciated, this is only the second time I have ever tried to do vacuum test's, I'm sure this is evident, and a few pointers would help me in tracking down an ongoing problem with the process of elimination.
Thanks for reading my post.

The EGR uses vacuum which will cause a drop in manifold pressure, however I think it should be closed at idle and open at about 1500RPM's. Id look for a leak or a loose line fitting assuming your guage is good and properly connected.Whats the problem you are trying to diagnose, misfire,rough idle, or dropping a cylinder at idle? (might be sticky/dirty EGR)

Manifold pressure drops on accelleration,and returns somewhat as the RPM rises, but with carbs and some TBI's, venturi vacuum ports are available. Vacuum rises with airflow and RPM's at venturi ports. The MAP senser is not connected to a Venturi Port.Think of it this way.Vacuum advance distributers needed a strong dependable vacuum supply to advance the timing as the RPM increased.Since MAP drops on accelleration, vacuum pressure reduces and won't work there for distributer advance. As airflow increases, it produces a stronger vacuum signal in the Venturi area because of Volocity. Vacuum is Vacuum.Therefore there is a difference in the two ports and readable vacuum sources. When looking for exaust restrictions, the MAP senser vac line seems good.