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Engine ping, hesitation, backfire like sound

Engine

Model: 993, Year:1995, Mileage:146,00, Type of use:Both Street & Track
What I am experiencing is a pinging noise, which sounds like a timing issue to me. The noise is somewhat intermittent. I recently ran an autocross and it it occured fairly consistently shifting from 1'st to 2'nd and when I stepped on the accerator. On a straightaway, If I apply power evenly it doesn't seem to appear. I climb a fairly long grade on the highway and the ping occurs if I apply power. If I maintain power, it doesn't seem to appear.The check engine light is not coming on. I have replaced the plugs, plug wires, caps, rotors and air filter.The mechanic I use thinks it might be carbon build up, I have swithched to Chevron and have used an additive he recommended. This does not seem to have cured the problem. Another mechanic I talked to said it is probably the belt between the distribuotrs.Do you have some ideas of possible causes? Or ideas of diagnotic tests to run?Thanks!

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