I had a 'lopey' idle for a while, but ignored it because I wasn't driving the car often. It ran great, only on idle it nearly stalled the lope was so pronounced.
I finished replacing the water pump, stat, cooling hoses and stuff the other day, was inspecting the hoses for leaks, and happened to notice a vacuum line running off the canister that wasn't connected to anything.
I found the other end, put my finger over the end that was pulling, and the idle smoothed out. HEH! Free repairs teh best!
Those cheap flexible connector "sleeves" that GM used to make line connections can also become very soft with heat and age. In the space between the line ends, they can collapse under the vacuum and obstruct the line.