Dont trust the reading of the stock oil pressure gauge.
I had a similiar prob when i had my idle issues with my sard 550cc injectors. Everytime the car would go into spastic idle and go back to normal idle the oil pressure gauge used to just go to 0 and sit there and sit there for ages till i turned the car off and on again, but the oil pressure was normal while the gauge said 0 pressure everytime.
The stock gauge is also said to be very slow reacting to a proper electronic oil pressue gauge.