How would I check the timer?

The timer can be checked easy. Just remove the fat red wire from terminal RH connect a meter lead to that terminal.Put the other lead on any other timer terminal, move the timer knob around. As it makes and breaks when you hear clicks if you don't get any reading then the contact is burned. But burned contacts are NOT intermittant.Thats why I say the breaker needs attention. They get hot on the heat side of the breaker and open. The motor runs on 120 volts so it is not affected.

dear sublime master .in my 35 years of servicing appliances .i have seenn numerous dryer timers misbehave in the manor i suggested especially g.e. dryer timers . i have turned on dryer to minute cycle ,opened door ,held the door switch in started timer +slowly turn, observing heater ,ever so slightly jiggle timer knob to see heater come on +then go off.that said ,customer never said breaker tripped+then reset . remember according to customer sometimes the dryer works perfectly ,explain your breaker theory now

I have found numerous dryers with every part on them working wonderfully.
The timer has even been replaced on a lot of these. I have seen these dsame customers buy new dryers only to have the problem be electrical. It very well could be the timer. But most of the time intermittant probs are not related to the timer. And since the timer is not refundable youare screwed if you plop down good money on one and that ain't the problem.
Take the dryer to another location and try it if you have checked out every other posibility. That is all I am saying. Your dryer drys perfectly AT TIMES and then other times it takes a dump. That is an electrical problem.

I just fixed my kenmore gas dryer model 110.87984110 with a three dollar part.
Cheap Home Repairs
This site helped me diagnose and fix my dryer which saved me from buying a new one! :D