2004 XLT 4.6 V8 133k miles. Last Friday while merging onto the freeway, the tachometer started fluctuating (vibrating) rapidly between 2500 to 4000 rpm and the truck would not accelerate beyond 50 mph. This happened several times within half mile while applying steady or no pressure to the gas pedal. Transmission shifts were smooth. I took city streets the rest of the way home.

I had the dealer inspect it this morning but they could not duplicate what happened. Check engine light never came on and it didn't throw any codes. They think it's an O ring in the transmission but since it didn't happen during their test drive they aren't sure. They want $900 to open the transmission to verify it's a faulty O ring. They also said if it is the O ring, there's only a 10% chance it'll cause additional damage.

Any ideas what happened? Since I only drive the truck 25 miles per day, I'll see how it goes between now and the next service at 135k miles.

Thanks.