You will have to find a model number for any help on a specific unit.
Perhaps the following will help you find it.
Model Number LocatorIf I understand you correctly every 30 days or so the cooling gets so bad that you have to do a manual defrost and it then runs OK for another 30 days approximately. 30 days does seem like a long time for a defrost problem to show up, usually it is about a week but it depends on the humidity where you live and how often the doors are opened.
If I am correct on he above then there are 3 things that can effect defrost.
1. Defrost timer. This assumes that your unit is older and uses a timer. The timer is often mounted in the control console or under the unit at the front or back. If in the console there is usually a hole that will allow you to force a defrost cycle.
2. Defrost heater. Usually under the evaporator coils.
3. Defrost thermostat. Clipped to the defrost coils.
First remove the evaporator cover in the freezer so you can see the coils.
Do not let them de-ice.
If they are heavily iced/frosted over you have a defrost problem.
If yes.
Manually force a defrost cycle by turning the defrost timer cam till the fans and compressor turn off.
Now check the defrost heater to see if it is on.
Be careful you do not want to burn your fingers.
If the heater is on then the timer needs replacing, re: it is probably stalling during it's rotation so is never getting into a defrost cycle.
If not on.
Unplug the unit.
Remove the wire for one side of the heating element from the wiring and measure it for continuity, usually around 20 ohms or so.
If the heater is OK
Remove one wire to the defrost thermostat and measure it, should be 0 ohms when frozen. Note that it opens just above freezing so must be frozen to check it. Also inspect it, if it is bulged at all replace it even if it measures OK.
If both the above are OK then odds are the defrost timer contacts are toast.
Best way to test this is a live test to see if you have 120 volts across the heater/defrost thermostat combo.
If it is an adaptive defrost board or defrost is controlled from a main unit control board you have to find out how to force a defrost.
If you do not own a meter, I would suggest you purchase a one. You can get a decent digital multimeter for under $20.00. You do not need fancy though it is nice if the leads are a couple feet long.
If it saves ordering one unnecessary part it has paid for itself and you end up owning a useful tool.
Most places will not let you return electrical parts so if you order it, you own it.
A couple things to watch when measuring ohms and continuity
1. Always remove power from the machine otherwise you could blow your meter.
2. Always disconnect at least one side of any device you are checking. This eliminates the possibility of measuring an alternate/parallel circuit path.
3. When checking for closed contacts and continuity use the lowest scale (Usually 200 ohm