From your description it sounds like the a/c compressor clutch is engaging and disengaging frequently. If I was in your place, I'd look at the 12V signal coming to the compressor clutch by measuring the voltage at the clutch. If the voltage is not constant then I'd look upstream and trace the circuit to see where the supply to the clutch comes from. On the other hand if the voltage is constant and the a/c clutch is engaging and disengaging, then I'd look more closely at the compressor itself.
In tracing the problem upstream, I'd have a look at the fuse box where the a/c relay lives. Usually in the fuse box there is a relay that is energized which in turn sends power to the clutch. If I found the voltage at the clutch intermittent, then I'd have a close look at this relay. In simple relays there are 4 terminals. 2 are for the coil and 2 are for the load. I'd first look at the control or coil side of the relay (terminal 85 & 86) and see if the voltage is constant there or not. Voltage aapears here when the a/c compressor is commanded on. I'd then look at voltage at the other two relay terminals (87 & 30). Terminals 87 and 30 should have voltage present all the times when the switch is turned on regardless of the a/c switch position. If the voltage is constant at 85/86, then I'd say the relay was at fault internally and should be replaced. If the voltage at 87 & 30 is not constant, I'd look at the fuse which feeds this relay and the fuse box earth points to make sure there is a good earth. Now all this I am saying is based on a very simple a/c circuit. Your Alto being a modern car may have other logic for the a/c control. If the voltage at terminal 85/86 is not constant then I would need to investigate this problem which could be in the a/c control box. But without knowing the layout of the Alto, I cannot go any further. But at least by following the above, you would have ruled out certain components and zeroed in towards a direction. Also gain knowledge about how the a/c control works by doing a bit of googling. Knowledge is power. Good luck.
