Something dragging the circuit down??

Hi, got a bit of a random problem with the electrics in my GLi cab with a mk2 8v engine.

The good points, everything works. Turn the ignition key so the battery lights on but not starting the engine and everything works fine.

Bad points, engine struggles to start (always had this and with your foot on accelerator it'll go ok). Once engine is going you can run all lights and heater on full (although full beam will make it cut out) but after about two minutes the engine struggles and the battery light comes on before dying.

I've started the engine with no lights or electrics on and there's 14V at the battery and the alternator is giving out about 12/14V but then after about 2-3mins the output from the alternator starts dropping off and the engine goes to stall, give it a blip with the accelerator and it'll be fine again for another 2-3mins before doing the same. The alternator I bought new a while ago and only fitted it a couple of months ago.

Anyone got any ideas, as would be greatly appreciated. Could it be the alternator or something else dragging the circuit down?

your not going to like this but

Dead alty.

check the gearbox earth strap