2000 GMC Sierra Z71 5.3 engine miss

My truck is experiencing an engine miss during a gradual load on the vehicle (for example: traveling up a slight incline on the interstate in overdrive) it begins to miss until it drops out of OD or I step on the gas it clears up. From idle to cruising speed I don't have the miss, only when I experience the above conditions. I am not getting a message center alarm and it has not shown up on the diagnostic computer. It's beginnig to get worse, and still no message center warning light. The message center window is operational. Help !