I drive a '88 Grand Marquis, 5liter w/ air. In town driving I don't have a problem, but......?

at highway speeds, when going uphill, or simply accelerating on the highway, the vacuum seems to go too low and the baffles that direct air shift to defrost and the thermostat for the heater core opens to add heat. I had this prob a couple years ago and added a can of oil conditioner to raise compression. It worked. But now the prob is back and that can doesn't help. Where to look - what to do?