98 Sienna Transmission problem (at 170K)

I have a 98 Sienna at 170K, and recently I am experiencing transmission problem (still run very smooth, but strange behavior).
I have the oil change every 3-5K, ATF change every 20-30K, timing belt at 110K, and spark plugs at 165K. Otherwise, no any tune up or maintenance.
The problem occurs at 1500-1800 rpm at around 45-50 mph.
If I accerate from lower gear, it goes up to OD smoothly without any hesitating. But, when it gradually slows down from OD to the problem range (as stated above), I can feel something happens like engine misfired (it does not feel like clutch worn out).
Someone told me to disconnect the battery to reset the electronics control, but the problem still exist. I know if clutch worn out, the problem would appear in may circumstance, but this one does not.
Can anyone give me some advice on how to troubleshooing or repair?
Thanks

I would suggest you to get the error code first. That would tell you if anything is wrong. Then start from there.
Sienna's transmission is not good. I have mine replaced at about 25K miles under warranty. Toyota had extended the warranty to 7/100000 for certain years and models due to a lot of early stage trans failures.

I don't have any advice, but would interested in hearing what you find out. MY 98 had a little trouble getting into a higher gear around that speed the other day, but the van was still alittle cold. Once warmed up it was and never happened again.

Good Luck


Ozzie