I have a whirlpool dryer, model number WED 5100 VQ1 that starts sometimes. The circuit breaker is fine. The electrical outlet is fine. The lint trap and dryer vent are clean. I have not tested any of the electrical components, yet because I need to purchase an ohmeter. However, I notice that when it does not start the drum light is very dim. When the light is bright, the dryer starts without any problems. Before I start disassembling the dryer to test fuses and such for continuity, I wonder if the fact that it starts sometimes might be an indicator of what component might be malfunctioning?

You have a thermal safety partially opening due to dirt. You will have to take it apart and test the thermal fuse near the heating element and the heating element. You will be able to see the heating element and you can see if it's burned in two. There are also fuses at the ends of the element that will need testing, too. You will have to clean the drier out thoroughly or it is all for nothing. Every air duct will have to be removed and scraped and washed out to remove baked on lint deposits which is like a concrete lining. This will burn so the manufacturer has installed the safeties to keep from having a house fire. The safeties are oval-shaped with a round center part containing the safety with two plug-on terminals for the wires and when they burn open from the dryer being dirty nothing will work. Using an ohm meter you put a lead on each terminal of the safety with wires unplugged and if it's good you will have a reading of about zero; if it's bad it will read infinity. Driers were the number one cause of fires at one time, hence now we have safety fuses. Look at: http://www.youtube.com/watch?v=MMP7BW1lLs4 to see how. Also: http://www.youtube.com/watch?v=iBiRw-KElC4 ; http://www.youtube.com/watch?v=yFTZGgVisCI
http://www.electrical-forensics.com/Dryer/ElectricClothesDryers.html
http://fixitnow.com/wp/2010/10/09/whirlpool-dryer-model-le5800xsw3-wiring-diagram/
When you finish cleaning the drier and install a new safety it is best to also replace the belt, too, because you do not want to have to disassemble it all over again and if the belt is over two years old you should replace it.

Note that on most of the videos it does not show them washing out the parts, but this is important because it's the only way to really get the parts clean, just don't get motor wet or the capacitor if it has one. Vacuum out the motor. I would move the drier out to your garage to have plenty of space to work. You also want to check your vent and clean it, also. The vent should have a short run and be straight as possible. The job is really easier than it sounds. There is no shortcut that will work; it has to be cleaned well. Before you re-assemble it make sure to use white lithium grease (high-temp grease) and grease all bearings. Just vacuuming out the dryer a little does little good. DrTom4444