Exactly right -- they can do all sorts of other stuff too. It will torment you by passing all of the tests and not showing any problems if you bring it to an autoparts store to test on a machine. Then one day -- when it's the worst possible moment, it will fail and leave you stranded. Yeah, it happened to me before.
Any auto parts store will have the TFI module. Check to see if you need any special screwdriver bits to remove it. Before you spend the $30 or so, make sure the cap, rotor wires & plugs are good.
TFI's suck! I had one go out on me while racing a camaro! lol Was right n his ass, then all of a sudden the tack flyes over to red then down to 0 and power is gone! Had to pull over. left me stuck on the side of the road! My buddy came out and we swapped TFI's, worked great!
Then this past fall, my car sat over the winter. I had it started this spring, ran ok...pulled the motor, replaced it, when to start it..no spark. Swapped distributers with my buddy..worked great! Damn TFI!