Around 50 or 55 mph, my 2001 Toyota Camry just began shifting up and down right around that speed. It shifts up and down quickly and jerksback and fourth. It just started this morning, but this afternoon, it didn't do it.

First thing I would do is to check the fluid. Second thing is to get the codes read to see if a system has failed.

as tenspeed asserted, there could possibly be a transmission fluid leak, would take a look at the flexible lines leading to the radiator, these rubber parts often go to hell with constant exposure to transmission fluid. look for red, then you'll know to tighten the clamps or replace the rubber there~

OK. When I checked the fluid, it was up to the mark and it was red on the white napkin. I had the fluid changed probably about 6000 miles ago, hasn't been long. I'll get the code readings from Advance Auto. I can say though that no dash lights came on. Is there something else I can do? I noticed one of your colleagues mentioned a "Throttle Position Sensor," to another subscriber regarding a similar problem. Is that a possibility? Thank you all so very much for your help.

TPS would throw a trouble code out there, yes diagnostic scan, buy one yourself for not too much money~

Toyota needs correct fluid ,worn out already maybe if not correct,or rear speed sensor going out.

Some scanners can detect pending codes that have not happened enough to turn on the CEL.

Gentlemen, thank you all so much. As Judge Roy and Tenspeed have advised, I am going to get a diagnostic scan as well as have the fluid checked by a technician to make sure it isn't worn out as Jimmy 7 indicated. Thank you all again for your help. This may have saved me thousands of dollars.