It sounds like the keyboard on that machine just doesn't like caps lock
pressed with another key. That's rather unfortunate if it's a hardware
problem, and not some software doing it.
Do you have another keyboard you can plug in and test? That might not
rule out software completely, though.

Here are some things you can do to get a modifier:

1. Find your insert key. It might be fn+m for numpad insert, or some
other key, and use that.
2. If you don't have one or that's inconvenient, then you can use
SharpKeys to remap something to insert.
Right alt might work, if you have one. You'll want to find a key that's
easy to hit, and you can't use it for its normal function after you
remap it.
3. Write an addon to remap some other key as your NVDA modifier. This is
similar to the SharpKeys solution, except you could get the original
functionality of that key by hitting it twice.

You can also use the Input gestures dialog to remap NVDA functions to
other key combinations which don't need the NVDA modifier.