Slipping transaxle

I just had a fluid and filter change on my 99 Elantra,104000 miles. I have been fairly faithful in servicing the transmission. I believe the specified ATF was used as my regular mechanic told me he had to use
Hyundai specific ATF when I set up the appointment. The car rode fine for about 2 weeks now there is a wicked slip from 2nd to 3rd when the car is warm and under torque. I love that car and hopefully won't have to sink a bunch of money into it. Comments,suggestions?
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Check the fluid to be sure it's full. If not, take it back to have it corrected-- they failed to fill it correctly or it's leaking. If it is full, then you probably have a serious problem. The good news is that, as transmission replacements go, the 1999 Elantra is relatively inexpensive -- I'd expect $1500 to $2200 depending on location and how much they'd like to hammer you. The factory list on Hyundai remans has been $1508 for years, but the dealer cost is so low, most sell the trans for around $1000.
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