Vehicle Shudder problems of the 2009 Toyota Matrix

When going up a hill in 4th gear car shifts down to 3rd gear to maintain speed & there is a loud noise that shakes the car (as if the engine's going to fall out). Happened about 20 times so far. Toyota dealer mechanics got car to repeat this 3 times (once hooked up to a computer). The mechanics & the service manager all feel that the transmission needs to be replaced. I have 100,000 mi. Extended warranty. The person from Toyota (not from the dealership) that has to approve putting in a new transmission refused saying it is an intermittent problem. He then said it was the way I use the gas pedal. Then why did it happen on the 3 roadtests when brought to dealer. The shift into 3rd (on a hill) is so violent that I'm afraid to keep up speed on the parkway when going up a hill. I pull into the slow lane & put on my flashing lights & stay at 50 mph or less. I feel that the car is unsafe to drive. I have 90,000 mi. On it. I have reported this incident to Toyota customer relations on the phone (they are based in California). They said they would look into it & get back to me in 3 days (tomorrow). I have bought 4 Toyotas in a row & got my sister-in-law & her son to each buy one. If this is how Toyota treats a loyal customer (all cars are serviced at Toyota) then I guess I & my family won't be buying one again. I paid a good deal of money for the extended warranty. A lot of good it's doing me. Thanks for listening.