I have been using Airtel Broadband service for 6 months by now. I have subscribed to a 4 Mbps plan, and connection is stable always. My wifi router remains switched on for days without any break, even then no disconnection ever faced while download in progress.
Only once I had some issues with my broadband not working, but the problem was sorted out soon.
Overall, Airtel ISP is one of the best in terms of connectivity, reliability, price, and customer support