To bring a bit of joy in to your world; I had this problem with my Spider about 2 years ago - got progressively worse, sometimes starting sometimes not, played with the aerial, moved the steering column, put in a new battery, used the emergency start procedure - etc etc you get the idea.
Eventually after 6 months the car gave up, the code light came on and wouldnt go out - that was it. In the end it was diagnosed as a broken immobiliser - had to have a new one fitted
As far as I remember (although its been a while, so dont take it as gospel) the immobiliser has to be coded to the ECU and then to the keys, its not a cheap job.
I had an indie garage do mine, he also changed some relay that he found to be duff - total cost was about 700quid and it took ages (4+ weeks) to get the new immobiliser from Italy.
Not pleasant reading I know, but the problem hasnt returned.
From the sounds of it the immobiliser in your car is dying, all the work arounds described here are simply delaying the inevitable.