Problem with 2001 1.2 Fiat Punto starting?

Hi,

Ive a 2001, 1.2 Fiat Punto. Since getting a new alternator installed in October ive had a problem with my car starting, ive had several new batteries, which have all tested perfect and no joy. The mechanic must have swapped the original battery with another by accident when he serviced it and ive had the problem since then, he stuck a piece of wood in to hold the battery down.

Recently, i was driving along at like 50mph and everything switched off, i mean the speedometer went right down and then it all went back on again with the time clock back to 0. One minute it will start fine and then i drive it to a shop or petrol station and it wont start again. (it's bad when u have to carry a spare battery and jump leads in the boot)

Can anyone please advise me on what the problem could be and how to fix it. Thanks!