FYI, you MUST run at least 91 octane in that car (93 if you can get it). Do NOT run the 89 as it can act as you describe and can be very harmfull to the motor. The last thing you need is a chipped piston (new motor time). S/C cars have to have premium fuel. Do the plugs as FFDP said and then get some more 91 in there minimum.