EPPENDORF 5415C lid latch keeps cycling on and off when plugged in

It's probably power failure starting. Check the two electritic capacitors on the left on the PCB board. If they are bulging, leaking, or even appearing normal, you want to change them. Mines were rated 820uF at 25V (the service manual says 470U/25V). The lead spacing is 5 mm.

I had the same issue with my unit, but ignored the problem and kept it on. I ended up frying other components on the PCB board. Besides the capacitos, I needed to replace the transformer and the weird-looking 5V voltage regulator (obsolete, but still available from China through eBay) and patching several traces. You don't want that, defenitely.

Good luck!

I'm wondering if there's a read switch on this unit that detects if the lid is closed and consequently latches it shut?  If so, it may be flakey.

Yeah, there is a switch in the lid, which prevents the rotor turning without closing the lid. If the switch malfunctions, you can either start the rotor without closeing the lid or it doesn't start at all, I presume

I'm not sure if the switch is a necessary part for the locking mechanism. The latch solenoid is energized in idle by unregulated DC supply that feeds the 5V regulator for the microprocessor. Once you hit the button, start the timer, or turn off the unit, the solenoid de-actuates and locks the lid. The solenoid cycles erroneously if the voltage fluctuates or drops near the operating voltage of the solenoid. That's my guess.

An easy solution would be to remove the latch bar from the lid and plug the hole with a pipette tip or something to bypass the solenoid and the switch. My lab mate used to do this trick as he was impatient to wait for the rotor to completely halt and the lid to unlock. But this is far from ideal if power is failing. When 5V for the microprocessor fails as well, the unit will become inoperative completely.

Ken is most likely right and even if he's not it's still a good idea. In time electrolitic capacitors (that always use liquid dielectric inside) trend to dry out and loose their function. I'm not sure when 5415c was put on the market but it's not spring chicken. So replacing them can not be a bad thing. They can have a higher voltage and if they are part of stabilizing the power they could also have higher microF values. 

I suspect, when Ken is talking about the voltage regulator he's referring to that big square thing on the lower right with many markings none that yielded anything decent when I searched. That would feed the processor I think on the lowest right hand corner pin.

Good luck 

Adrian