Your problem is most likely in the motor, but before we go there let's rule out some other things that are inexpensive and easy to check.
1. Unplug the dryer and check that all the connections from the cord to the connection block are tight. Then plug the dryer back in and make certain you have 120 volts at each line into the connection block.
2. If there is no problem there, then unplug the dryer and take off the top (the screws are inside the door frame) and take off the front (two screws hold it on). Clean out the cabinet and the vent are, and in particular the area in the blower and the area around the motor. Put it back together (be sure and reconnect the door switch) and see if that fixes it.
If these do not work, your culprits are the motor, timer and start switch. I would replace the start switch (you have about a 20% probability that this is the problem based on my experience) since it does not cost much. If that does not solve the problem, you will have to consider spending big bucks on the motor, wihtout knowing for certainty that will fix it but knowing it is highly likely.
Well, good luck and let us know what you do and what you find out.