Strange lumpiness after print is about 1cm high

I have a home-made Prusa 2 which has been printing fine for years. I recently dusted it off and since it was in an unknown state I reflashed it with the latest Marlin. The electronics is RAMPS 1.4 and all seems fine. I printed out thin wall and 50mm cube no problems (3mm ABS), but when I print larger pieces it starts getting very lumpy after around 12mm height. Specifically it shows on some wall sides but not others. I have tried printing with plenty layer cooling fan and no fan, different bed heating, cool ABS and hot ABS, I have checked the extruder for skipping stepper and jamming filament all with no change. It's not simply the height triggering a dodgy connection because the 50mm cube prints perfectly.

I have printed many tens of Kg of filament over the years but never seen this, neither can I see it in any of the many troubleshooting guides online.

Any suggestions very welcome indeed!

Now, if I can only attach these images right... The tall cube is ok, the other thing is borked.

Thanks,
Phil

Edited 1 time(s). Last edit at 09/28/2018 07:29AM by davies46.

Still trying to get to the bottom of this... Here's another image which shows it well. Granted it doesn't start off well, but after a while it seems to just lose the plot!

Slic3r 1.2.9 did the chopping so I might try another program. I'm attaching the config bundle in case anyone's that interested

-Phil

Edited 2 time(s). Last edit at 10/01/2018 05:54AM by davies46.

Some ideas: Perhaps try Slic3r Prusa edition 1.40.x. Then set the extrusion multiplier to 0.45. Also make sure autocool is on with with the time limit at 40 seconds, and minimum speed at 10mm/s.

I'd also check that your PLA is OK. It's hydroscopic. If is taken up water then it spits when hot. You said you've returned to 3d printing after a break? Where was your PLA?

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Using ABSPrusa Mendel Zaphod with Pronterface and slic3r 1.3.0. Printing well with 3mm PLA and ABS through 2 x J Head Mk IV b and Wade Geared Exruders. Controlled using RAMPS1.4 board running Marlin_v1.1.9