My problem started a couple months ago but it was much more severe. Its for a 1990. It would misfire throughout every driving range and especially acceleration. Right away I changed the spark plugs, cap and rotor. I also changed the fuel and air filter, and ran techtron through the fuel system. After I did this it started working significantly better. Doesnt miss on acceleration anymore, but it still does at idle speed, only once the engine has warmed.

Anyone have any ideas what could be causing it?

A miss at idle that goes away as soon as the throttle opens is almost always going to be a lean misfire caused by a vacuum leak. The mixture is richer when the engine is cold, which may overcome the leak until it warms up.

What you have to determine is if it's a solid, rhythmic miss of a single cylinder missing, or the nervous shuddering of the whole motor idling too lean.

A single cylinder miss you can determine by simply pulling plug wires one at a time until you find the one that has no effect on idle speed/stability.

A whole-engine lean condition will stabilize and idle speed will pick up if you partially choke off the air coming into the motor. Place your hand over the intake pipe and slowly close up your fingers over it. A lean engine will show improvement in idle at some point as you choke it off, an engine with a proper mix will just start to bog down and get unstable, if that happens, your mix is good, something else is wrong.

If you suspect an air leak, you can spray water from a spray bottle all over around the intake and any vacuum lines, when the water finds the leak, the motor will react by smoothing out momentarily.

There is also the possibility you have an injector that's not spraying a good pattern at minimal delivery warm idle conditions. This can create a lean misfire condition in one hole too...