1978 2.3 idles very rough for long time

as of late i drive on the freeway for twenty to thirty minutes uninterupted and even when i get to my work and i idle in the parking lot my rpm guage drops down to 5 and shakes my whole car and threatens to die off.. if i started it up in about a half hour after that it would idle properly at a little over 11. the weather is not cold at all i live in Arizona. anyone have any ideas?