Compression mystery 944S
 

I have a 1987 944S Porsche (in well cared for condition as I have owned it for 14 years). The problem is that occasionally the number 1 cylinder compression drops to 0 lbs... then, after 8-10 minutes of driving, it returns to normal. Any suggestions? My mechanic thinks it's the rings sticking, but isn't sure without tearing down the engine.