Sanden 508 compressor, how much psi?

[Houston Ben]

Hi all,
I just installed my AC (high mount sanden 508) and filled with R134a. After driving for about 45 min on highway and when I started to come to a stop with RPM's between 6-700 the sanden would vibrate so bad that I could feel it in the cab and hear it but only at the above mentioned rpm's. I am wondering if I have too much 134 in. I put in till the psi was at 250 psi high side and around 50 on the low which is 3 cans and one can of 3oz oil since I lost some the first time I tried to put 134 in.

Any help would be appreciated.

Ben