For the past two summers, my pot and window-box plants have all died. Plants such as stachys, thyme and pelargoniums start happily enough but after a few weeks a grey powder appears on the leaves, then they go brown, frazzle from the bottom up and, ultimately, die.
The grey powder could be mildew, which usually weakens or debilitates a plant rather than killing it rapidly. Whatever the cause, all diseased and dead plant debris should be removed and burned. Make sure your plants are spaced well apart to increase air circulation, water at the base and do not overfeed. Your description of the plants frazzling from the bottom up and dying makes me wonder if there is a rot, or Botrytis cinerea, a fungus that causes grey mould. All the plants that you mentioned like a well-drained soil - too heavy a soil and/or overwatering can cause rotting. Water plants only when they need it and, again, remove diseased debris to prevent the spread of infection.