1988 Festiva dies on idle

Last night I took her out on the highway to see if maybe the Seafoam and higher RPM's with a load on her would maybe work it out. I drove about 10 minutes away and when I pulled over, she purred like a kitten. As I drove around, it would sometimes idle smooth, sometimes a little rough. I'm going to burn through this tank of gas and try another double dose of this stuff to see if it's just junked up in there, but my buddy mentioned it could be an idle solenoid that is sticking. Thoughts?