The garages you have used sound like they should be avoided: Gearboxes, dizzys and leads never cause hunting and never will. Hunting is always caused by too lean a mixture but is rarely due to a carb adjustment error. When you engage gear, the engine load increases and increases the depression in the intake manifolds.
You have an air leak somewhere in the intake side of things which, it's more complicated on your model as it's a V6 so there are more places to look. Prime suspects are the intake manifold gaskets and possible hairline cracks in the manifolds themselves.
Any decent garage should be able to diagnose and fix this without any problem at all, but on the V6 the labour costs can be a bit high if much needs to be dismantled. The good news is that if you're an enthusiastic owner then you don't need to go anywhere near a garage...
Join the Granada Owner's Club
(see link: unfortunately I've just found that their site is currently incompatible with FireFox, but works fine with Internet Explorer); they're a very friendly and knowledgeable bunch and will always offer advice and practical help to an owner in trouble. The 2.8 Ghia X is always a model which is of especial interest to them. Annual membership fees cost less than half a tank of petrol, so it's an absolute bargain.
You should soon have your problem fixed and be able to enjoy your car: I used to love mine back in the day and it was a rarer variant with the optional sports handling pack. It was fun to have a big car which would run rings around the hot hatches of the time and leave them behind in a cloud of tyre smoke!
I hope that you've found my answer to be of some assistance and ask in return only that you return the favour by rating my answer.