I'm having issues with my Volvo, and I can't quite figure them out.
Maybe someone can help?

When I'm accellerating (or when I'm not), it feels as if something's
pulling back on the car, or like it's not getting fuel for just a
second.  Almost like if you were running, but someone had a string
tied around your waist and jerked on it every so often, causing you to
slow down but not stop completely.

The impulse generator went bad, we replaced that.  We changed out all
of the spark plugs and wires (it was time for a tune up), and the
battery connection is fine.  I don't think it's electrical.

We went through the fuel system; we checked the line, the fuel
filters, the pumps, the fuel pump relay, and all the wiring - it's all
good.   I can't really figure out what it might be.

Anyone got any ideas?