Our TH magna had its transmission serviced about 6 months ago. It was playing up whenever the gear changed and it would overrev. Mechanic changed spark plugs as well as servicing the transmission. Been fine until last couple of weeks where first gear is overrevving slightly. Seems to be ok in the other ears. Car is automatic.

Over revving?

Do you mean - "shift flare" where the engine revs up between gear changes?
or
"loss of drive" where the car is in gear but the revs go up all by itself and the car doesn't go faster?
or
"over revving" where the engine revs go too high in first gear before it changes gear?

I would start by checking the fluid level and asking the mechanic what fluid he put in there.

Thanks Mad Max, I would have to say it's over revving and I tried to listen more carefully today, I reckon it's doing it in the other gears as well but probably more pronounced in the first gear. It was worse 6 months ago so I assume that what the mechanic put in was ok because the transmission was smoother after he serviced it but the last few weeks I have noticed that it's struggling slightly.

At what RPM does it change gear when driven normally (let's say pressing the accelerator around 30% down) My guess is it should change between 2500 and 3000 rpm in this scenario.

Does it feel like AS it goes to change from first to second gear that the revs suddenly lift higher like it's momentarily in neutral between gears? (Shift Flaring)
Or does the gear change seem normal and smooth?

There is a way to "reset" the transmission computer, this 'may' correct whatever the exact issue is that you're having.

http://i.imgur.com/hc4B0gL.jpg

Thanks Shamous, I'm really not sure how to best describe the problem but it seems to struggle and take longer to switch gears. The fix you mentioned looks easy enough to apply so will give that a go.

Thanks Shamous, I'm really not sure how to best describe the problem but it seems to struggle and take longer to switch gears. The fix you mentioned looks easy enough to apply so will give that a go.

Perhaps this is an engine problem and not a gearbox problem?

Get someone who knows what they are doing to drive the vehicle.