It could be anything from a loose wire to a defective motor. Using a multi-meter is the way to find which. If you don't know how to read the schematic on door and use a multi-meter,you'll need to hire someone who does.

intermittent problems are hard to diagnose. I'm not a fan of guessing.

If the problem disappears a multi-meter won't do much other than tell you what you can already see. Therein lies the problem - you can't trouble shoot a ghost.

You can either wait for the failure to become bad enough to have it diagnosed by a pro or replace it. Or have the pro start guessing at what parts to replace next if the furnace doesn't fail while they are there.

The gamble is the repair(s) may cost more than had you just bought a new one.

How old is the furnace?

Found the problem- turned out the 20 pin connector is disconnecting as it vibrates i guess. i tried crimping the female end of each pin of this connector with no luck. I have it with tape holding it in a position that works (so far) until i fix the actual pins by the diagram. While testing each point according to the diagram with my voltmeter, the switch started cycling so i pushed the back of my hand and voila! was able to reproduce the clicking\"fixing" the connection. Like i said, still need to find which pin isn't touching and replace the pin i guess.... Thx.