Salvia shriveling up

[bag927]

Can someone please tell my what is happening to my salvia greggei(spelling?). In the last year I have lost 3 plants. They are mature plants (3 years old), but they get yellow leaves, shrivel up, and then all of the leaves fall off. This only occurs in sections of the plant, not all at once. Eventually, the entire plant, save one poor little stem, dies. I am left with sticks in the ground.
I use an organic fertilizer and do not overwater. I have been gardening for many years, but this is stumping me. I do have clay soil which I am in the process of ammending with compost and 'Medina' soil activator.
Last year I noticed some very interesting bugs. They weren't your normal garden pests. The nymphs were pinkish in color, the adults were gray w/ some green. I haven't seen them this year though.
I called a garden center that specializes in organic gardening, but they didn't have any idea w/o me bringing them either the bug or sending a picture. They are over an hour away, so I didn't do it.
I am really getting frustrated having to pull a plant every few months, especially one with the reputation of being hardy in the Texas heat.
Any suggestion or comment is appreciated.
Rejoicing in Jesus,
~L