Truck Forums - 98 Sonoma-cold air when engine loaded down

Any time I load the engine down (like going up hills, which there are a lot of here in WV) it feels like the air conditioning turns on and comes out of the dash vents. It might be just the ambient air coming in, kind of hard to tell this time of year, but it definitely changes distribution settings.
Normally drive with heat distribution set on floor level & defrost but it happens regardless of the distribution setting (discounting the AC settings obviously). If set to the dash vents the output air just changes to cold.
No other symptoms of anything wrong, power is good, engine runs smooth, no change in gas mileage, nothing else goofy in the cabin.

I did change the intake manifold gaskets about three weeks ago, and in the process replaced one disintegrated vacuum line (about 6" long) that ran from a "T" to a canister in the left front fender well near the firewall. Don't figure that would initiate this problem since the line I replaced was disintegrated to the point of being an open line (should have been happening before I would think).

My guess is that I have a vacuum line collapsing with the increased vacuum that goes with the loaded condition of the engine. Having come to that conclusion:
1) Anyone know if I'm headed in the right direction with this?
2) Is there a vacuum line in the engine compartment I might have disturbed that would cause this?
3) If not, how does one get at the lines going to to the climate control?

Any help would be appreciated, still have a lot of winter left so I'm going to have to do something with it.