check compression? i have experienced kind of this issue, it was a customers car not mine. i checked compression and the numbers were all crazy and kept changing. he had carbon buildup on the valves so bad that they wouldn't shut and kept putting out the spark. if the motor is old and the previous owners didn't use 91/93 octane all the time it "could" technically happen. After i confirmed that it was getting compression but it was all over the place. the same cylinder read different every time i checked it. i kept cranking the car with another car jumping it, crank for 10 seconds wait 5 seconds crank for 10 and i did this 2 or 3 cycles and finally the motor warmed up enough to shut the valves and start the car. ran it for a little while then poured small amounts of seafoam into the brake booster vacuum hose. it broke down and cleaned off all the valves and it runs great now. it's a 2.8 V6 audi.