Odds are that the defrost thermostat has gone intermittent.
I would replace that if the heater still checks out OK.

Another possibility would be a bad connection to the heater/thermostat from the timer but I would go with replacing the the thermostat first.

Here are your parts
Replacement parts for FRIGIDAIRE FRS26ZGHW0 Frg(v3) / Refrigerator | AppliancePartsPros.com

Here is a tech sheet
http://manuals.frigidaire.com/prodinfo_pdf/Anderson/218909400.pdf

Looking at the wiring diagram and the picture in the parts, it looks like your defrost should come on for 30 minutes every 6 hours of compressor run time.

You did not say what troubleshooting you did to determine that the timer was your problem. If you just felt this was the most likely cause and replaced it then your problem may well be the defrost thermostat or the heater itself.

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 (Item in Section ) 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. They have been known to go intermittent.

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 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 ohms). Then try higher scales. This scale is 0 to 200 ohms so if the device you are measuring is 300 ohms this scale would show an open circuit which it is not, you are just measuring outside the scale's dynamic range.

There is a good STICKY at the start of this forum about it's use.