Speed Sensor problems of the 2006 Mazda MAZDA3

Vehicle has had, on several different occasions, been driven, not at any particular speed and lost 4th gear, the engine light appears, an at light appears, and the brakes are applied to downshift the car down to 3rd gear. Upon me noticing this. I wait until it is safe to stop at a red light or stop sign and make a complete stop shift the car into park and back into drive. This seems to fix the disappearance and removal of the 4th gear as well as the at light, but only occasionally. It does not however remove the engine light. Though after a few different trips on different days the light will not appear on start up and does not show up until this issue happens again. The car shows error code p0715 and is possibly needing a new transmission speed sensor. This issue has not been fixed.

I was driving along and suddenly a loud noise, the check engine light came on and the car shifted from 4th to 3rd without losing speed (except from me taking my foot off of the gas when hearing the noise) all at the same time. My mechanic said the diagnostic machine pointed to the "turbine speed sensor", so this was replaced within 2 days. The same day I got the car back, I drove it about 30 miles and it did the same thing again. Now my mechanic is checking into the transmission, possibly causing the problem.