air conditioning turns off & on

Yes it's common. Problems with the HVAC unit have been covered on here loads if you do a search. Try 'hidden fuse' or '348 HVAC'

Here's a copy of a post I made on another forum.

"1. Check the 'hidden fuse'. The A/C system has a 30amp inline fuse holder that melts, giving a poor connection. To take a look : Open the front and remove the black metal trim from around the air/blower inlet. Pull up a couple of feet of the bonnet rubber seal from the area around the right bonnet latch. Carefully lift the carpet liner and you will see ECU/relays/fuses. Find the 30A inline fuse and inspect. If it's knackered replace it with a quality holder (the type the ICE boys use on big stereos).

And, if you are confident in doing this (or know someone who is):

2. Disconnect battery. Carefully remove the A/C unit - Remove gear knob and gate. Remove screws in ashtray and the console will come out. Make a note of where the wires go before removing plugs.
Working in a low static area (no carpet for example) open the control unit (small phillips driver). Visually check for dry joints. Resolder as required.
The unit has more than one board inside which are interconnected with pins - these connections are often poor. Carefully disassemble one board from the other and clean the pins with fine emery paper, reassemble with a little electrical contact grease. Do not use wire wool to clean the pins.

Good luck. It is often the case that the above work cures the problem.
"

 

I've had that problem with mine, and all that it took to fix it was applying some Stabilant 22a on the Ground pins behind the driver-side air vent next to the door.

There are a couple of pages on it here: http://webpages.charter.net/aircover/348/1994Ferrari348Spider.html#AirConditioning with at least one picture of how to remove the vent in a different section, as well as one picture of the common ground pins shot with the vent removed.