Why is my car struggling to change gear?

I would be really greatful if you could please give me some advice regarding my Renault Clio Automatic gearbox.

I have noticed for the last month or so that the car is not changing gears very easily and I cannot get up any speed without really high revs, the problem has got steadily worst and I am unable to get the car above 45 mph today.

I have had the fluid checked out, I have been told that it is the torque converter but have beeen advised to buy a whole gearbox as there is more chance it will be succusful.

I have found the part reconditioned but wanted a second opionion on the length of time to fit as I have been told it will take 8-9 hours labour for which ever option I take ie new gearbox or new torque converter only.

Also, before I invest in repairing this problem, is there anything else that may be causing the problem which will not be fixed by the new gearbox?

I look forward to your reply

Thank you