Spongy gears

Hi I am new to this.
I was wondering has anyone had issues with gears not shifting correctly?
By this I mean going spongy and won't travel up or down the cassette so the cable isn't indicating too tight or not enough tension.
I have changed the inner and outer a number of times and re indexed the gears so that it is running sweet up and down the cassette.
This fine for a few weeks then out of the blue gear changes start to stutter and when I try to re index the gears shift are either hesitant or non existent both shifting up or down.To correct this I change the inner and outer and hey presto gear changes are sweet.It is a cross bike with cables running along the cross bar and down the wish bone to the rear mech ( not like traditional setup where the rear mech cable runs along the drive side chain stay.Problem just occurred again yesterday and I changed the cables over with the exception that I added clarks sealed cables (zero g with dirt shield with the hope that this will help).
When I removed the cables I noticed that a little grit and water had gotten in it had been a particular wet week and like I said the cable are facing towards the heavens due to the routing.
This said re lubeing is only temporary as the problem just re occurs.I am running full shimano 105 5700 10 speed.
I have done the work myself been robbed by my lbs just to see if I was doing something wrong,on the second visit just got the spring in the mech has gone so it isn't pulling as It should,as I have explained new slick cables=slick shifts.
I have ridden through the winter and use the bike 5-6 days a week and accept maintainance is partand parcel but new cables every month or so is tedious and frustrating when you are riding half expecting the shifts to go to pot any time soon.The plus side is it isn't too expensive couple of quid here and there it is just inconvenient!
With the evidence of water incress it appears just at the rear not at the brifter end I gave in ordered some middleburn cable oilers in anticipation to again see if it helps.
I have written this post to see if anyone out there is suffering/suffered the same and if there is any bright ideas or fixes to improves the longevity of my shift cables instead of them being doomed to the bin.Speaking of cables I have used shimano outers and inners which are supposed to be compressionless Clarks cable kits and local bike shop off the reel cables (so to speak)so cable quality I think all spectrums are covered unless I stump up for a jagwire cable set which have less compression.I have focused on the rear cable area to be the problem and have tried a short loop and I am now running a longer than normal cable loop like on a pros road bike (supposed to provide less friction ,just what I have read).
Hope this covers all bases sorry about my rambling but I have tried to explain in detail so I don't the it's just cable stretch etc (cause it ain't as far as I can tell).
Thanks in advance Nathan