Exactly what it says on the tin - unfortunately it's a stability problem for more intensive uses. There was a previous fix done but more recent versions of Groovy from later cores no longer handle classes int he same way and the hacks to purge its memory-leaky ways need to change.
Now that we have gotten the "dangit groovy!" bits out of the way, there's an easy fix! I would suggest incorporating the latest version of the fixes Pipeline is using to work around the issues from here: https://github.com/jenkinsci/workflow-cps-plugin/blob/master/src/main/java/org/jenkinsci/plugins/workflow/cps/CpsFlowExecution.java#L1052
It should be a pretty much direct copy+paste, thankfully, and the above code is already pretty heavily tested (you can port over the tests too if'n y'like).