Turn off everything! - Computer, DSL modem / router, hubs or switches. Wait for 20 to 30 seconds. After waiting turn the DSL modem on first. Wait about 30 seconds or so and then turn on any hubs or switches and the computer.
Depending on your DSL modem model, the SYNC light maybe label as "DSL", "SYNC/PPPoE", "WAN", "LINK" or "Ready".
RF ( Radio Frequency ) or EM ( Electro-Magnetic ) interference nearby the DSL modem or wiring can cause the connection to bounce. Neon / fluorescent / halogen lights, fax machines, speakers, etc. can cause these problems. Try moving those devices or move the DSL modem itself.