from near perfect to rubbish prints

Hi all,
Please can someone help, my homemade printer was printing, as I considered near perfect, printing out most of the Prusa i3 parts .stl files until I ran out of fillament. I aquired a new roll of 3mm ABS fillament, loaded it into the machine and have printed nothing worth saveing since, I have cleaned out the extruder nozel checked the PTFE feeder tube cleaned the extruder drive wheel adjusted the idler tension and varied the temperature from 225deg (previous fillament settings) as high as 265 degrees and everytime the print starts out fine for 2 to 3 mm high and then the layers start to separate and look like areo chocolate as per the atatched picture the left hand side pic is from the old filament the other is an example of what I have tried to explain. The gcode file is the same file as printed ok with the first fillament.

I can't tell much from the fuzzy, small picture, but if you're using the same gcode file that may be the problem. Filament diameter varies from one spool to the next. If the new spool is different size you need to reslice unless you're using volumetric extrusion. In either case, you have to measure the filament diameter and multiple places (try 20-30) and calculate the average value. Use that value as the diameter of the filament when you slice or when you print.