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2001 Nissan Sentra GXE transmission won't shift

I have a 2001 Nissan Sentra GXE twin cam 16-valve and sporadically (about once every three or four days), the transmission seems to be stuck in third gear and won’t down-shift or up-shift. The ATF levels are good, I check them often and I called the dealership and gave them my VIN to see if the recall work for the Crank/Cam position sensor has been done to which they’ve replied that “No outstanding recalls exist for this vehicle”.
How do I remedy this?
Also, is there an after-market all-metal dipstick for the sentra? A common problem is that the plastic top gets brittle and breaks off in the tube. I don’t know who would put a dipstick tube so close to the exhaust manifold.
Thank you!