valve adjustment

Hey guys, I got an 88 c1500 350 5.7
I just replaced the valve stem seals and adjusted the valves and it runs pretty good until about 20 minutes of driving. After the car has been at full temp (190 degrees) for about 10min, while waiting at a red light, the truck will idle a little rough. The rough idle will become increasingly stronger the longer i drive it. Its not enough to cut off the engine, but I can definitely feel it. It is timed at 0 degrees. Also when i replaced the seals, I used an umbrella type seal on the exhaust valve which wasn't used on the original design.(it was a recommendation) It helps reduce oil being sucked into the exhaust valve by the vacum. It drives fine after I start driving from idle. If i misajusted a valve or a few valves then why doesnt it idle badly from the start when the engine is cold? What other things besides the valves can cause this sort of problem? Please help

Thanks,
Matthew

Hi, Sounds like your valves are tighting up after it warms up (metal expanion) try readjusting them with motor warm.