Mk6 Ford Fiesta St - Revs Dropping When Switching Down To 1st/2nd

Hi!

New to these forums and i have a concern about my car which I though I would put to you guys and gals.

Bought myself a Ford Fiesta ST Mk6 57 plate back in September '14 and loving it. Did have one problem, kind of early, which was when I would go to work in the morning the battery would be dead, or would die on me very quickly if i was in there listening to music with the engine off. Gave the car to my girlfriends step-dad who is a MOT engineer who did some power tests? on it and determined the battery was dead. Got a new battery and everything seems to be fine, hasn't died on me since (although I'm now paranoid about having electrics on too long without the engine).

Recently i have noticed that when changing down to 2nd and 1st the tachometer would suddenly drop down to about 700, quite quickly, causing the lights to dim and then would shoot up to about 1,100. This doesn't happen all the time but mainly when i pull into my driveway; i have to slowdown considerably as its a tight turn with a high curb. This doesn't happen when changing between higher gears just when dropping down to 2nd and 1st. It also happens when reversing into my parking spot, revs would drop, lights would dim and previous owner has fitted an annoying reverse sensor (that doesnt work), this would also make a dying noise. I combated that problem by giving the car extra revs when reversing.

The worst time this has happened is when i pulled into my driveway and my revs dropped and rose, i took the car out of gear to roll the rest of the way and the revs kept going up and down until i came to a full stop.

I have had a look through the internet and as you can imagine i have found all sorts of different causes, belts, alternator, battery, battery cables but i wanted to see if anyone here has had a similar problem or knows what it may be?

The car has only done 50k miles so i would like to think a new alternator is not needed?

Let me know your thoughts. (i will get an engineer to look at it as soon as i can)

Thank you!

Dev.