Hi
This is a computer controlled transmission. This means, most of the time, these types of problems are caused by failures of the internal electronics of the transmission. Such as a shift solenoid or, shift switch assembly. The shift switch assembly, as an example, sends messages to the main ECU to advise it of the gear the transmission is operating under at the time. Best approach is to have your transmission's electronics scanned for any possible problems. If this checks out ok, then you likely have something worn/wearing within the transmission. Don't be sucked into a full transmission overhaul that you may not need. The autos in the Commodores are quite robust and can take the biggest abuse, unlike their Falcon counterparts. Try and source out a repairer that's prepared to fix ONLY what needs doing instead of a full re build. If you need any further specific info, I currently have an add running advertising my services. I'd be happy to answer what I can. Chris