Backfiring when engine hot
 

I have a 1974 911 with 91K miles.

Recently, the engine has been backfiring when it has been driven for a while and fully heated up. If I don't get it back to the garage quickly, the problem gets progressively worse until the car sputters quite a bit and will eventually stop running. If I let it cool for an hour or so, I can start it back up, although it will still sputter a bit. Give a few days and it runs fine again until it gets heated up. Then the same thing happens.

Seems to backfire and sputter the most in the 3,000 RPM range.

I thought it might be too much oil so I pumped out a bit but that hasn't helped.

Suggestions?
Thanks