Blob Artifacts on Prusa i3v ABS Print

Hi,

I've been printing for a few months now with a Makerfarm Prusa i3v, and for the most part prints have been of acceptable quality. I'm finding however that on the occasional print (it seems to be fairly inconsistent), at a seemingly random layer the print has issues maintaining the straight profile, causing the blob-like overflow seen on the neck of the picture attached. It also happened slightly further up on this print, but as you can see the print was fine until the neck, had serious issues, then continued to print slightly off at higher levels. I haven't tried a reprint yet, but in the past when this issue happens, if I reprint with the same settings I don't see the issue again. Does anyone know what my problem could be?
My thoughts at present are that maybe the hotend is making contact with the previous layers while it prints, causing them to melt as the new layer is applied. While I think this makes sense, it doesn't really explain why it randomly happened 3/4 of the way up the print, then automatically corrected itself.
I'm printing ABS with an extruder temperature of 240 degrees Celsius (which is in the recommended 220-260 degrees Celsius for this brand of ABS).
Apologies in advance if this problem has been addressed elsewhere, I'm just not really sure what title to search for it under.
Any help anyone can give would be greatly appreciated.
Attachment 10533

It looks like you might be getting too much heat in the neck area. If you notice the area in the next is pretty small so a lot of heat can build up there. Do you have a fan on your printer? ABS likes being kept warm while printing, but I noticed having the fan on can help for narrow/small areas in a print.

You didn't mention the name of the brand, but 240 C is pretty high for ABS. Have you tried playing with a lower temperature? Maybe try starting at 220 and going down, or up depending on the results, in increments of 5 to 10 degrees.