cambo - It looks like you might be using my sketch from the Arduino forum. I have had some false readings due to background light levels using RISING in the interrupt and have had better responses using FALLING. I am using a light to voltage sensor TAOS Photodiode part number TSL267-LF from Mouser: http://www.mouser.com/ProductDetail/TAOS/TSL267-LF/?qs=rBDwU0K8fVpc2GQ0WesZ9g%3d%3d. It gives a nice dependable square wave but is very sensitive to ambient light so I have had to put a cover over my meter to keep light out. I have been using the Stalker for recording power use for several months and it has not missed any pulses. Here is a example of a daily usage chart:
I bought a JYE Tech oscilloscope and it has helped me a lot for looking at the signal. I would recommend getting a oscillscope if you are going to do much with electronics, it would help for looking at the signal from the solar shield and might help you solve that problem. I have some pictures of the response so I will post them if I can get them uploaded.
I see that a new Stalker has been released using the ATmega 328. Once they are available in the States I will update the sketches using the increased capabilities of the 328.