Sorry I don't have pictures, but time is tight and it's hard to photograph the way this problem works.
When your shifting gets mildly erratic with STI road shifters, the problem is often bad housing. The inner wires push through the ferrules so that the housing does not provide consistent resistance to the cable moving through it. However, a more sinister problem can have expensive consequences. If you examine your housig and DON'T see collapsing ferrules on the ends, look at the shift cable itself inside the shifter. It can fray in there, causing gradually increasing drag. It gets expensive when the cable degrades enough to spit broken fragments into the shifter itself. These can jam inside the inaccessible mechanism badly enough to ruin the shifter in some cases.
If you find a frayed cable, don't panic. Click the shifter to the highest gear position for the rear or lowest gear position for the front. Undo the anchor bolt at the derailleur. Push the cable just far enough for you to decapitate it with cable cutters. Pull the end out. Then, without pushing the shift levers at all, use a small magnet to pull out any leftover bits of wire.