oil pressure sensor

I bought an oil pressure gauge from Dragon gauges, it is digital. it has a sender that is the same size as the stock oil pressure sensor. I removed the stock unit, wired it to ground to avoid the dash warning light. I installed the gauge sender in place of factory unit, on first start up the gauge worked beautifully, I **** the car off, then on second start the gauge has improper readings, being very erratic and having the warning light on. I know there is oil in the car and it is at the proper levels. I use the recommended synthetic weight oil. any ideas on why this sender would work once then not give correct readings. ??????