Erratic rpm's

after my car warms up, and driving down the road the check engine will come on, but before it does my rpm's will either jump way up making the car speed up or they will go down below 500 and almost kill the engine. Now when checking the codes on the ecm it flashes 12 once then 13 once repeating over and over. I have replaced the TBS already.
Give me a little help. BTW the check engine light is not the one flashing, it is the seatbelt light, but the engine light does come on while driving.

I also forgot to mention the the smell of sulphur that happens just before the check engine light comes on.