Ever since its introduction in TEBC the variable 'checkInterp' has been worse than useless: due to a committer's mistake (boo to me!), it is never reset to 0. So we are resetting it to 1 on every DECACHE and testing it, all for nothing.
Attached two different patches: one uses checkInterp correctly, the other eliminates it altogether. After careful timings one will be chosen for committing.