What might cause engine shudder at initial throtle opening?

I replaced the parts because soon after I bought it late last year, I noticed this condition and performed catch up maintenance. I replaced all the ignition parts becasue the installed parts, with the exception of the spark plugs, were all original. I scanned it to be sure, but it is not setting any input/output codes. No, it's not motor mounts, it is throttle related. I operate a one unit on call service business, so my truck is full of noisy tools. When the shudder occurs, the entire truck vibrates until throttle opening is increased. There is another poster here with the same problem. It only does it on initial throttle opening during acceleration enrichment. Beyond that throttle position, it runs perfectly.