Hissing when the compressor goes on (or off?) implies low refrigerant. I'm sorry to hear that you switched from R12 to R134a. I got the SDL with R134a and it can't hold a candle to the cooling ability of the SE with R12 which is doing the hissing thing (leaky schraeder valve).

If you search the archives, there's lots of discussion on the various trips and triggers of the AC compressor. Could be that you have dirt in the clutch gap or a misadjusted gap.

Sixto
91 300SE
87 300SDL
83 300SD