This is kind of an interesting circuit... What's setting the frequency of oscillation? From what I can see, it's the feedback loop combined with the low pass RC filter on the voltage divider input to the comparator.
Maybe why your seeing less oscillation when you directly couple the mosfet gate to the comparator is because the comparator doesn't have the drive capacity to switch the gate as fast (only 16mA from the datasheet), and therefore it's an extra time constant in the circuit stopping it going into high frequency oscillation.
Either way, I personally would much prefer to use something like a nice and cheap TL494 controller IC that'll give you a fixed frequency output with PWM, soft start etc. It also has a bandgap 5V ref on chip so you could get away from the regulator as a reference. If you haven't seen them, have a look at the extensive application notes on the web. They're an IC that's been around for ages and is in a huge number of computer power supplies. ~$0.30 from digikey last time I checked.