That is a very narrow range and your amperage draw could be high with it cycling on and off so much. Usually you want one degree or more between the low and high. Also why are you using Min Time 005:00 Then OFF that is telling it to stay on for 5 mins then go off. I think your intent is to not have it stay on longer then 5 mins which I get but temp should turn it off you would probably have to add a Def line to achieve all that. Kinda overly complicated but I do see where you were going with that.
I would set like this
If Temp < 77.5 Then ON
If Temp > 78.5 Then OFF
If Temp < 30.0 Then OFF <-- this is in case the temp probe fails and gives you a false reading (will read in 20s if fails)
* This will maintain your temp within 1.2 degrees of 78
could be a faulty outlet as well, I'd try this test.....
remove this line.... Min Time 005:00 Then OFF
plug an light into the heater outlet.
manually turn the heater on and off, if the light goes on and off the outlet looks OK.
grab a cup of tank water and a cup of cold RO, heat the tank water over 80* (microwave for a few seconds) put the temp probe in the cold water. Apex should read the colder temp and turn the light on. Move to the heated tank water, temp should adjust and turn the light off. If the temps on the display don't change drastically the temp probe is bad, if the temps change but the light doesn't respond as expected there may be some other code keeping the heater on or off.