1985 Toyota Corolla Rough Idle

This just ocurred two days ago, I turned on the car and it was idling very rough, even after the reaching normal operating temperature. I just changed the spark plugs, checked and filled the coolant to the proper level, checked the spark plug wires, and checked and filled the engine oil to the proper level last week. Today I drove it for about 30 minutes and it ran just fine, after those 30 minutes it started to idle rough, and it got rougher and rougher as more time passed by, not teribbly bad, but bad enough that I noticed the change. However, when I am driving it feels smooth, and I can easily reach speeds of 45 - 50 mph.