Strange lumpiness after print is about 1cm high

[davies46]

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 share this image right...

Thanks,
Phil

[iDig3Dprinting]

A picture would be handy, lumpiness could cover all many of surface anomalies.