Why would the blower motor on a 1997 Escort stop working for a few weeks then work for months and then not work again for weeks?

Without a lot of hands-on troubleshooting testing, I can only speculate as to cause and must generalize in this answer. The quick, down and dirty answer is that the blower motor reguires electricity [power] to operate, and lack of power is the cause of the failure[s] to operate. The situation of sometimes working and sometime not, suggests some kind of INTERMITTANT[sp?] failure of the electrical system [SWITCH(s), wires, connectors, fuses or circuit breakers, AND the wiring, connectors, and brushes INSIDE the blower motor itself] to CONTINUOUSLY supply that power. In your question, the absence of mention of a blown fuse[s] tends to eliminate a "short" circuit being involved. This then suggests the more likely cause is related to an intermittant "open" fault somewhere in the circuit. An open fault occurs when a conductor[wire] is cut, broken, or a connection becomes loose, disconnected, or corrosion builds up between the contact surfaces of the connection/connector. Generally, corrosion acts as an insulator, reducing or even totally blocking the flow of electric current through a connection. Intermittant opens occur because the nature of the fault changes. In most electrical intermittants, the primary cause of the changes is VIBRATION, which tends to make and break the contact, sometimes allowing current flow, sometimes not. The fix for this type of issue is to find the faulty connection, and correct the cause, such as cleaning the contact surfaces, and securely remaking the connection. This means checking all connectors [for proper assembly, lack of corrosion, and evidence of overheating(which enhances corrosion)], all wires [for evidence of breaks or cuts of the condudctor(s)which sometimes may be concealed by the insulation]. Sometimes, corrosion between the contacts within a connector is not extensive enough to see without disconnecting the connector. Often, the simple act of disconnecting, and reconnecting, a connector will scrape and dislodge enough corrosion material to "clear the fault," curing the problem until the corrosion builds up again. When disconnecting automotive wiring harness connectors, it is IMPERATIVE that the LOCKING TABS on the connectors NOT BE BROKEN OR DAMAGED. There are usually two tabs, on opposite sides of the connector. Pros use a special tool to retract these tabs but, WITH GREAT CARE [and four hands!} they can be safely retracted using two VERY SMALL screwdrivers. If, after finding, disassembling, cleaning, and remaking all connections in the blower circuit, the problem still exists, then the probability increases that it the problem MAY be inside the blower motor itself. The same factors discussed above can occur inside the motor. Unless water has gotten in the motor, corrosion is generally not a factor. BUT, the operation of ANY motor using "brushes" results in wear of the brushes. The resulting wear material from the carbon brushes is a black, burnt, fine powder which coats everything, and when it gets on the commutator ring terminals it prevents good electrical contact between the terminals and the brushes. That poor contact acts as a resistance to the flow of electrical current, and can make the motor run raggedly, or shut it down altogether. Vibration [always present in moving vehicles] can knock that wear debris off the commutator, allowing the brushes to make good electrical contact with the commutator ring, and thus allow the motor to begin running again. This could be another explanation for the intermittant nature of your problem. If your electrical supply system [from the vehicle battery all the way to the blower motor connector] is "cleared] of involvement as to cause, then the motor is probably the cause, and the best fix is to replace the blower motor. Some motors are capable of being disassembled, and therefore being overhauled or rebuilt, but unless you are capable of doing that, it would be much easier to simply replace it. Matter of fact, an expensive but quick way to find out if the problem is in the wiring circuit or in the motor, is to replace the motor FIRST, and if the problem goes away, then it WAS in the motor. If the problem remains, then the electrical circuit is the culprit, and you WILL HAVE TO go through the entire troubleshooting drill, in additiion to having UNECESSSARILY replaced the motor. I hope this helps you to understand, and correct, your problem. Good luck!