I have the same Mercury Topaz '92 and I JUST replaced the air flow sencers. I can't remember which one it is but its one of the two. Whoever told you is exactly right on dot. Its very easy to access and replace - its right on top. Also check into the oxygen censor I just had to replace that and the motor mounts...lol. Honestly you and I are in for alot with our cars...I'm having the same difficulty I'am hoping to buy a Ford Taurus instead
I wish you luck in your car.