From the ArcaMax Publishing, Gardening Newsletter:
http://www.arcamax.com/news/gardening/s-1902-183484
Question: I have a troubled variegated geranium. It seemed happy for a
while, and sent up lots of new flower shoots. Once the flowers went
by, I removed the spent blooms. However, lately the plant has begun
yellowing all over. I repotted it in new container with new potting
soil; I've also tried altering the water and/or light it receives.
Should I have left the flower stalks on? Practically everyone in my
neighborhood grows geraniums with little or no effort! What can I do
for my poor plant?
Answer: Geraniums do best with full sun (a little afternoon shade in
really hot summer areas), a little less than average watering, and
minimal fertilizing. You were correct to remove the spent
flowers--pruning them off was not the cause of the problems.
It's possible you are "killing it with kindness." With too much water
and/or too much fertilizing, geraniums quickly succumb to root and
stem rots. Once they become infected, there is little you can do to
stop the infection (including repotting in clean soil since the
infection is in the plant's system).
I suggest giving the plant as much sun as possible, holding back on
the water significantly (but not to the point of desert conditions),
and cutting off any soft, mushy, brown-to-yellowish stems (dip your
pruning shears in a 10 percent bleach solution between each cut).
Hopefully, it's not too late.