It gets stranger, if more useful. The real problem with blowing out of the interrupt timing was the
'n = Value DIG i ' statement (which I notice I hosed up posting the source code.)

I moved the "@ INT_RETURN " statement up a line at a time, and when I got above the "DIG" statement, timing was magically as expected. No amount of dinking with the statement and retrying the syntax, limits, whatever would stop this.

When I split the "Value" variable into three digits of one byte each and used an IF array to make decimal counting work in the main loop, and changed to a select statement for the pre-carved-up digits in the interrupt loop, it started working just fine.

Either there is a vaster subtlety than I can see, or the "DIG" math function is broken in some way that interacts with timer1 interrupts.