The biggest complaint I get is no heat or intermittent heat. The number-one cause of this is air in the cooling system. Whenever someone comes to me with a heater problem, the first thing I check for is air in the cooling system. Here's a video on how to purge the air from a cooling system.
How To Bleed A Cooling System:
Pressure Testing Cooling Systems:
Solving 'No Heat' Problems:
If you purge the air from your cooling system and you get your heat back, don't consider it fixed just yet. There might be a leak somewhere causing the air to enter the system. Here's a video showing how you can find coolant leaks in your cooling system.
Once you've found and repaired the leaks, your heat should be restored and you can go on your way. Read the Overheating Problems article to get more detail about finding and repairing cooling system issues.