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Mark, have you tried twiddling the contrast pot?  Usually a square blue thing, at the back of the display.

At one extreme of its adjustment the display disappears, leaving only the backlight.  Maybe it's set on the borderline and the tiny voltage change when the Pi is running affects the display.

Thanks Steve. I've not tried adjusting the contrast - mainly because it looks OK when the Pi isn't booting up.

I'll give it a go though, but I was more concerned about the flickering backlight - just wondered if it was something others had seen, however it doesn't deem to affect the operation of the emonPi and to be honest it's locked in a meter cupboard anyway so not really causing much of an issue. 

Have you got another usb power supply and or a shorter cable you can test with to see if this is causing the issue. I have experienced this issue when using an underpowered power supply in the past. 

markp - I have the same Flickering LCD but it is not faint.  

Glyn - 

I tried a different USB power supply (an Apple iPad 2 USB supply @ 2.1 amps) with the original USB cable and the LCD flicker is still appearing.

I tried two different USB cables with the Apple USB power supply.  One slightly longer at 1.5 meters / ~60 inches and one shorter at 1 meter / ~40 inches).  Both cables seem to eliminate the LCD flicker.

I went back to the original USB power supply with both cables (my 1.5 meter and my 1 meter) - still no LCD flicker.  Seems like it may be the cable.

Hope this helps.

Some of those USB cables have ridiculously fine red and black wires in them for carrying the 5V.  From memory I was seeing about a 0.7V drop across the one running to my xbian RPi.  I ended up cutting the ends off it, throwing the wire in the bin, and soldering my own much heavier duty wire between the two ends.  That brought the voltage right back up to what it was reading at the wall-wart end.  Your observation that it doesn't flicker with the SD card removed is consistent with that, as a running Pi will draw a lot more current than a halted one.

OK, I ran an extension lead to the meter cupboard this morning and plugged in an iPad charger and the screen didn't flicker when powered with that. 

The normal supply is via 6-core alarm cable, 2 cores for 5v DC, 2 cores for 9v AC, and the last 2 cores run back to a DS18B20 to monitor the temperature inside the house. The total cable run is only around 3m through the wall so I'd not expect a massive voltage drop over this sort of length, though I guess it's the only explanation. 

Is there any form of regulation on the emonPi PCB so I could supply a higher voltage? If not I'll probably look to put something together with a DC-DC converter and send 12v down the cable before regulating it to 5v locally. 

Is the flickering screen likely to be causing any other issues with the accuracy of the measurements? There's been no stability issues with the Pi and it seems to run fine so I'm not too concerned really. 

The emonPi regulators down to 3.3V for the emonPi board and passes 5V directly to the LCD and Raspberry Pi. I believe the Raspberry Pi can safely handle 5.3V, I would not risk going higher.