fluctuating oil pressure?

[1970bowtie]

ok, i gotta question i put a fresh motor in my truck it has probably 200 miles on it. when first cranked in had 55 psi and 2000 rmp while breaking in the cam. now my oil pressure gauge bounces all over the place (ive tried a different gauge) and while driving back from town it went down really low. i pulled over and checked the oil and it was good. but psi wouldnt get over 25 driving back so i was babying it. psi is fine at idle it stays about 20-25. so does anyone know what my be the cause of this because i am lost