The LT20 doesn't like light loads, at least out of the box. When my Dad and I worked mine over for skeet shooting, we took a turn off the recoil spring and opened out the gas port a bit. Works great now with skeet loads, at the expense of beating itself up with heavy loads or slugs.
I'd give it a hundred shots or so to break it in, with a heavier load. See if that clears it up. If not, then it may need to go to a competant smith for tuning.
BTW, good choice. The 1100 LT20 is my favorite shotgun.
- Chris