2003 MPV misfires at low rev

[jaydeez22]

Hi there, I have an annoying misfire I am trying to diagnose.
If I am driving around town at low speed, all is fine, acceleration, desecration, all good.
If I am driving at a high rev and over 100kph, it also seems fine.
The misfire occurs when I am driving between 70-100kph at a low rev on the flat.
It feels almost like a light shudder.
If I approach a hill at that speed the car drops down a gear, and the revs go up, the problem goes away. It seems to be only at top gear at a low rev.
If I am traveling 80kph on the flat and it is misfiring, and I click the overdrive button, rev goes up, misfire goes away???
I have had the throttle body cleaned, tried a few injector cleaners in the petrol, replaced air filter and coil but still have the issue.
Any help will be appreciated.
Cheers
Jamie