Sounds like your spark is breaking down.
Could be plugs, a bad coil pack(s) or
ignition module (R31 > R33 S1)
Id start by replacing plugs, if it continues check the coils for arcing etc. Wrap them in amalganising (spelling) tape.
If that does not help chances are its your 15 year old coil packs letting go.
Theres a few options:
1. Splitfires (good alternative -not as good as new stockies IMHO)
2. Stockies (New) $$$$$$
3. Another after market solution (bosch, msd, AU falcon etc)