Double Down

a friend has a 35 fountain ex with twin 496HOs. lately he has been getting an engine alarm on the port engine after acceleration that puts the engine in guardian mode. if he restarts the engine or waits a period of time it will reset and he can run for awhile before it happens again. eventually after a number of restarts it will not alarm until the next time he takes the boat out and then the sequence happens again. does not happen at any specific RPM. he did notice that if he hits the rev limiter both engines go into guardian mode immediately. Any ideas on what's the problem?
all gauges read normal when the alarm goes off. engine and drive oil levels are normal, sensor wires checked for loose leads.
approx 250 hrs on factory engines.

thanks in advance for any help with this guys!!