Eddie Yeung this sounds like your limit thermostat is turning of the unit. Post some images of it disassembled so we can see what you see. Use this guide [guide|21499|commentid=417084] for that
=== Update (03/17/2018) ===
Eddie Yeung looks like the diodes (Zener diodes as well) have failed. Those are most likely part of the rectifier circuit for your heater. The only way to fix that would be by carefully desoldering those and take a look at them to identify which diodes these are. Again, do feel free to add images of those if you are having trouble identifying them.
+
On the same image that @jayeff pointed out, can you identify that there is a resistor or similar between the contacts?
Eddie Yeung this sounds like your limit thermostat is turning of the unit. Post some images of it disassembled so we can see what you see. Use this guide [guide|21499|commentid=417084] for that
+
+
=== Update (03/17/2018) ===
+
+
Eddie Yeung looks like the diodes (Zener diodes as well) have failed. Those are most likely part of the rectifier circuit for your heater. The only way to fix that would be by carefully desoldering those and take a look at them to identify which diodes these are. Again, do feel free to add images of those if you are having trouble identifying them.
Eddie Yeung this sounds like your limit thermostat is turning of the unit. Post some images of it disassembled so we can see what you see. Use this guide [guide|21499|commentid=417084] for that