Start replacing vacuum lines, the vehicle is 13 years old, and that engine
bay gets mighty hot hoses are probably starting to leak and or collapse.
Usually the air goes to the defrosts as that is the default setting(makes
sense, clear windshield is important). It could be the engine is getting
tired. Compression test called for to determine that. Timing chain could
be stretched throwing timing off and as such lowering vacuum, vacuum gauge
for that one as well as a timing light, watch for dancing timing, some
shifting is normal because of the electronic control, but at idle it should
be pretty stable. Find a good exhaust shop to check back pressure in the
exhaust system. Also how hard are you pushing it on the highway, or is this
an around town occurrence as well.? I have only had one vehicle do this, an
87 S-10 Blazer with a 2.8 V-6, throttle body injection, and only when I was
running her bottom off.

Whitelightning

I will start with vacuum lines, its got 115K on it, but runs great other
wise, its a V-6 not sure what size. this is around town and alittle
highway, to get around here, it takes highway driving to get to where you
want to go, ie like the other side of town ( and that could be 25 miles in
some cases) anyway, Van runs good, highway driving in never over 65mph, town
driving is like 45mph, in some places its hilly, ie the ups and downs that
cause the problems with the fan.

thanks all for the input, at least it gives me a place to start looking.

It's a 4.3, its the only one they used. In very early models the 2.5 liter
4 banger was an option, but soon dropped. 115k is just broke in. Check
the vacuum line going to the cruise control, it runs close to the exhaust
manifold. the hose from the PCV valve goes to pot, usually close to the
throttle body. also the vacuum hoses going to the valve on the charcoal
canister, both the idle purge line and the control line.

Whitelightning