NAT with only one interface

[zapov]

I'm wondering if it is possible to set up NAT for local network using only one interface on my BSD server?
I'm using OpenBSD, and have this configuration:

interface: vr0
vr0 dhcp - uses dhcp to get ip from cable modem
vr0 alias 192.168.1.1 - gateway for local network

ip.networking.forwarding (or something like that - i'm currently not on that server)

NAT
from vr0 on 192.168.0.0/16 to (vr0) - something similar

All computers, and cable modem are connected to switch.

The problem is, that it seems to me that network is crashing on this setup. I've came to that conclusion because, sometimes I can ping outside, but most of the times I can't (for example, from ten attempts only one succeeds, and when it succeeds it only works for short period of time - couple of seconds).
From OpenBSD server everything works fine.
Inside local network everything also works fine.
But when I try to surf using computer from within the network, I need to refresh page couple of times for it to load (and most of the time, it doesn't load fully).