transmission problem...

Posted this message to Focus group but got no response or seems like its down.
I have a '01 Focus Wagon 1.6 liter automatic transmission (4 spd, Mazda
developed transmission) that I bought new 5 years ago. it now has 48K Kms and always dealer serviced. At 45K Kms transmission fluid and filter were changed. The car is driven most of the time in city therefore it rarely reaches speeds of 80 KMH or above. Last week on a highway trip I noticed that torque converter lock up did not happen which would have caused rpm to drop 200 more. Same thing happened yesterday again. It used to cruise around 2200-2300 rpms on a leveled road when going 90 KMH but instead the rpms were never below 2500.
I tapped the brakes little bit to see if the rpms would increase but nothing changed. So, I suspect that torque converter does not lock up.. Am I right on this? If I am right, is it costly to fix? I am very happy with this car and planning to keep it at least 2 more years.
Thanks in advance,

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