I had exactly this problem!
Cause: Excessive freeplay in the 2-3 clutchpack, worn servo & pump.
Cure: Gearbox overhaul.
I also have a Corvette Servo fitted, Kevlar Bands, a stage 2 shift kit and last but not least, I had the heavy duty 9 plate 2-3 clutchpack from the Utes & commercials fitted.
(I wanted to tow a Caravan)
Results: It's bloody fantastic!
Now the gearbox shifts are firmer, but only when you sink the boot into it and its shift points are slightly higher.
Fuel economy is back to the cars usual 6.0 to 6.5Litres/100Kms on a trip (I get over 1000Kms a tank out on the highway, about 500 - 600Km around town) and the performance is right back up there again.
(The car has the standard 67 litre tank)
I'm rapt with it.
Why do I get such great economy you ask?, a few reasons:
1. I live in country QLD so I do mainly long trips.
2. New Engine + new gearbox.
And the biggie:
3. Cruise control!
Have fun!