Thermostat needle

Let me state my problem again with this.
Everyday I warm my car up for 5-15 minutes before driving. After
approximately 2-5 minutes of driving the thermostat needle starts to
slowly drop down to cold, the car down shifts by itself and the engine
roars loudly becaue the RPM's are so high when the transmission down
shifts. I took my 94 Mazda Miata to a Mazda dealer. They first said the
thermostat was getting stuck. They checked that and that was not the
problem. Then they did some type of engine cleaning (so they said)which
cost me $74. The final analysis was I probably had a bent valve, which
would have cost me $750 to repair. I elected not to repair, becaue this
was an educated guess, but I wasn't interested in taking a stab in the
dark for $750. Any ideas?