At first it was about 6-12 inches of combined cables and traces, now 4-6 inches, no difference.

So I got another unit and soldered thru-hole components directly onto the legs and everything worked fine for several programming cycles. At least I know the programmer can indeed program a 16F628A SOIC in-circuit (not that it should make any difference from a PDIP unit).

I will replace the oscillator and capacitors on the circuit. If that doesn't make a difference I will solder this 'working' unit onto the circuit.

Robert