You really should cross check with a desktop, often notebook manufacturers cut corners on serial ports, plus notebooks have their own idiosyncracies:

If your laptop has any power management settings, put it in to full power mode, or even try tapping a key or clicking the mouse while the download occurs. It might be slowing down the processor after a period due to lack of inactivity, causing problems with the serial I/O. Run your notebook off A/C power if you're not already.

Try a brand new programming cable as well.

If it's worth your bother ... the key is to isolate the problem until there's no question where it's coming from.