I attend an AG church and am told the AG churches are Pentecostal, but a distinction is drawn between AG and Pentecostal. What's the doctrinal difference?
There isn't a difference, in the way you have phrased your question.
Rather, the difference is that Pentecostalism includes many denominations, one of which is the Assemblies of God church.
The Assemblies of God (AG), is a group of over 140 autonomous but loosely associated national groupings of churches which together form the world's largest Pentecostal denomination.