The vent temp in my '94 Caprice also drops noticeably at stop lights. My theory is that the heater core is plugged enough that at idle it won't flow enough coolant to stay warm. Bring up the RPMs and it flows more. Since plugged heater cores are standard fair on B and D bodys, and it's been a few years since I last flushed mine, it seems reasonable. The only other thing I could think of is a worn out water pump not pumping enough water at idle, but that seems very unlikely because overheating has never been an issue.
Disclaimer: Any hokey-assness on my personal vehicles does not reflect the quality of my work done while I'm on the clock
Last edited by Bugman; 11-29-2014 at 10:14 AM.