JFW

My 94 Caprice has 190,000 miles and never missed a beat . I parked it for 15 minutes, restarted, and it had a misfire. In the cold morning air, with a cold engine, it seems to run OK, but once it warms up a bit, it misfires, especially heavily when under load. At mid to higher RPM it seems to clean up and run better. The dealer recommended a full tune up and to replace the distributor, but never really said this would fix the problem. I replaced plugs and wires, and it may have helped a little. One question. This car has the Optispark distributor. In my service manual, it seems that the distributor should be sealed, and hold a vacumm. Mine does not. I tested the venting system, and both hoses and the check valve work as they should, but by my interpretation the distributor should be sealed form the outside air, and mine is not. Is this causing the misfire? Thanks for any help.