Q: misfire on 1993 Lexus SC400

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my car messed up out of no where, when i took it to the shop they say my car has a misfire. the machine that they are using to tell where the misfire is coming from has not told them anything, my car idles really funny and shaky, my rpm line is bouncing between the 123 and getting stuck and the car is smoking out the pipes black smoke, i can drive the car but it rides really bad and it will not accelerate over 30 mph....
(2) Answers
Bad news is all I have i am guessing you have a tremendous amout of miles on this car. You probably have a cracked ring or a Valve guide seal leaking. hopefully only a valve guide seal leaking. Buy a couple cans of SEA FOAM engine Treatment and pour one Can per Tank of Gas and see if this helps. Good Luck