Sounds like an ignition problem. Take a look at the HT leads, coil and distributor. Check for any signs of corrosion which would indicate water/moisture ingress.
If you replace any of the ignition parts DO NOT replace with pattern ones, only genuine. Non genuine (i.e. non Lucas) dizzy cap and rotor arms) cause no end of problems on the V8. Bosch coils are OK and decent non-genuine leads.
