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So far, nobody has mentioned it so I will...
The EGR system is designed to lower burn temperature to inhibit the formation of oxides of nitrogen (NOx). It allowed manufacturers to program more agressive timing advance curves without detonation on lower octane fuels, ESPECIALLY at lighter loads where EGR has its greatest effect. Take away the cooling effect of the inert (non-combustable exhaust) gas in the cylinder without changing the advance curve (only an option with mechanical distributors) and you have exactly the problem you describe.
I'm not familiar with the exact setup on your 84, but to operate properly, the EGR valve should open partially under light engine load and further as you approach med load. At higher throttle is will start to close. This is NOT engine speed related which means you cannot simply pop the hood and open the throttle to check it - but if you observe the valve and *snap* the throttle open, you should see it move. It should be completely shut at idle.
Apply some sort of vacuum source to the valve while the engine is running and see if the valve holds vacuum and opens. With the valve opened, the engine should be idling VERY rough or die due to mixture dilution. If it doesn't, then it's probabably just the port/tube from the exhaust manifold that's blocked and in need of manual cleaning. A partly clogged passage can fool you into thinking it's clear but not allowing enough exhaust to pass to prevent pinging. Just confirm that opening the valve makes it run terrible at 1200-1500 RPM too.
If the passages seem to be clear then you may have to "tee" in a vacuum guage to the EGR hose and observe the it's getting vacuum while driving and go from there.
One thought if there's no vacuum - make sure the coolant is not running cold since the EGR is inhibited below 125°F coolant temp.
It sounds like you recently bought this car so check the emission certification label and see if it's qualified for high altitude (I call 1.25 miles in the sky high alt). That 10" manifold vacuum sounds awfully low and usually means a mechanical problem, but check with local shops to find out what normal readings are in your area. (This has nothing to do with being a Volvo, it's normally 18" Hg at sea level on ALL gasoline engines.) If it's lacking in power too, then make sure the the timing belt wasn't installed with the camshaft mark one tooth late. Improper engine calibration may actually be part of your problem.
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