You may have a faulty water control valve (if equipped) thats closing at low vac or the vac actuators inside the cabin are moving because of reduced vac. the opt-68 has 3 major parts....The one that concerns you in this issue, I affectionally refer to as the "bastard box"
...aka the a/c programmer. The BB manages and routes vac to the many actuators that control the vent system... It gets its "orders" from the control head, BUT, the system uses the BB as part of the loop for signals and permissions, so a request from the CH goes thru the BB to the BCM (blower control module >3rd piece<)...and the clutch is energized along with the appropiate vent doors opening.
You have the control head with buttons and display. Then the BCM for power distribution to the blower and clutch. This confuses many mechanics when the a/c clutch stops engaging because they find the wire to the clutch is still hot...but the clutch does nothing. The grounding is managed in the BCM as every other control function on a Corvette, and the circuit is completed in the BCM where it also lets the ECM know whats going on so the IAC can bump the throttle 50 rpm to prevent stalling when the a/c is running....
So, if you have a hot water control valve,. look at that (by the core underhood) and see if its sliding closed with low rpm, then do a check of the vac line to the cabin thats behind the distributer, driver side. Look for a check valve by the dist that also sends a vac line to the cruise control. The vac source is a nipple just above the fuel regulator on the rear of the plenum.
You don;t want to have to look at the BB unless you have to....:ugh Its rare to have a vac line break or some leakage in the cabin other than an actuator diaphram leaking but you'd know that by a function that did'nt operate well. Loosing heat sounds more like vent blend door creeping shut or water circulation.( heater control valve) BTW, I eliminated my control valve with no ill effects.
Also, there is an orifice in the heater line as it exits the intake manifold. That is there to maintain some tension on the water so it keeps the head and intake full helping to prevent hot spots. This can also slow the water flow enough at idle to keep anything from circulation in the heater lines. the top radiator hose handles most of the exiting water so when the system is not under a lot of pressure from circulation the water will follow the path of least resistance...the big hose. When the rpm increases there is enough pressure to move water thru the smaller heater hose. Notice that one is smaller than the other...typical for a supply and return hydraulic system. You may also have the oil cooler plumbed into the heater return lines below and under the a/c compressor. That also helps create more resistance so the water needs more pump pressure to move thru all that smaller plumbing. The typical Delco or rebuild pump is not very efficient at idle and barely moves any water...so add all that up and you're cold idling in traffic ! Its a double edged sword...a more efficient pump cools better so there is lower water temp so there is not much heat available even though there is good circulation at idle....the stock pump does not circulate as well thus allowing higher water temps....that are meaningless if its not circulating at idle. Catch-22.
