Intermittent transmission code
 

My mechanic is pulling his hair out. My 2002 Boxster is throwing an intermittent transmission (tip) overheat code, where the gear indicator lights begin flashing which the owners manual says indicates a transmission fault. But, the lights stop flashing after 10-15 seconds, then may start again in a while, then stop again, etc. Code indicates an overheat condition. This seems to occur at moderate stop and go speeds and has not happened at highway speeds. But, if overheated, why would the indicators stop so quickly? The transmission can't overheat then cool every few seconds. We suspect a sending unit, connection issue, wiring problem, etc. Has anyone ever had any such issue? Transmission was serviced a few thousand miles ago. Fluid level is correct. Any help would be most welcome. This problem began after getting the secondary shaft bearing replaced and as the Texas heat climbed into the 90's.