Blazer Problems

I have a S10 Uk Spec Blazer over the pond here in the UK, the car is identical to a American model except that we drive on the other side of the car...

I have had Alternator problems with the car its alternator failed last week, bought a new alternator for it and thats faulty too, they have similar but different faults,
The Origional one just stopped charging altogether, the new one flickers between 14v and 16v then drops down to 9v and back up again, all the illunination on the car dims and brightens whilst the engine is running,
Could this be a problem with the car electrics that is back tracking and causing the alternator to go duff or simply just bad luck with getting a faulty alternator???

Second problem is that over here PETROL is £1.09.9 per Litre ($9.79 per gallon) needless to say its dear to run these 4.3 vortec beasts, we convert them to Liquid Petrolium Gas (LPG) (its $4.98 a gallon) when i run on LPG the Engine Management/ Fuel system warning light comes on, switch back to petrol and after a few miles it goes off again, i think its the oxygen sensor as LPG burns so cleanly that it produces trace emmissions (would any one agree or shed more light on it)

Welcome to the site!

It looks like, as you've determined, the alternator is bad and needs replacing.

Concerning the O2 sensor, it is a thermocouple. It contains a metal that actually generates a voltage in proportion to it's temperature. This voltage is in the millivolt range and is read by the car computer.
If the O2 sensor is setting off the Engine Management/ Fuel system warning light, it might be temperature related. Most LPG systems have a heater to vaporize the fuel before letting it into the engine to avoid fuel line/equipment freeze-up. You might check the fuses for that and maybe during an Engine Management/ Fuel system warning event, stop the car and quickly check under the bonnet for frost covered lines, solenoid valves and or regulators.