Climate control modules can be problematic. Symptoms vary but include failure or persistent dead battery after not using the vehicle for a while.
Porsche 911 Problem Reports
Most Reported Porsche 911 Problem Reports
Ports and passages for secondary air flow can clog at the cylinder head/cam housing area. Partial engine disassembly may be required to fix the problem.
An accumulation of small air leaks at various locations around the intake manifold/intercooler area can cause performance problems.
Various fuel trim and oxygen sensor limit fault codes can be traced to an internal vacuum leak at the oil separator.