Like you said, it could be the SMC. To reset this: 1. Shut down the computer. 2. Disconnect the MagSafe power adapter from the computer, if it’s connected. 3. Remove the battery. 4. Press and hold the power button for 5 seconds. 5. Release the power button. 6. Reconnect the battery and MagSafe power adapter. 7. Press the power button to turn on the computer.
Otherwise it could be dirty contacts as well, try giving an air gun squirt in them if you have one.