I had a BBC do that exact thing a few years back, I noticed it as I was warming it up before a race. The pressure was good until it got around 4000 rpm and it would drop to nothing. I was young and stupid and thought it was just a bad gauge or sending unit and ran it like that for 2 passes. When I removed the pan, the pickup was laying in the bottom, broke off. Thank God for Mobil One synthetic oil, didnt hurt anything luckily.