2992 Vectra 1.8 Immobiliser woes

[sup3rbird]

I have a 2002 Vectra 1.8 SXI which is doing my head in. When I bought it about a year ago it had an intermittent fault which prevented it from turning over and starting. This happened about once or twice a month. Disconnecting the battery for a few seconds reset something and it was good for another few weeks. Since then, it has been happening with increasing frequency and has reached the point where it fails one in three tries. I'm getting a bit fed up with it as I use the car to make parcel deliveries six days a week and I'm afraid it will finally refuse to run at all. The engine light has been on since I bought it (£100) and it won't rev over four thousand but I don't mind that. I drive like an old lady to save petrol anyway. I've heard car thieves can bypass immobilisers and It seems wrong that they can make my car more reliable than I can. Any helpful suggestions gratefully received.
Cheers