pLysS freezer stock instability

[Frank van de Loo]

Dear methodists,
I find that my BL21(DE3)pLysS competent cells, and pET-containing
strains derived from them, lose their chloramphenicol resistance
during storage at -80.  The competent cells were grown in 
chloramphenicol and stored with DMF; they also lose viability
(very low after 1 year).  The pET-containing strains suffer dramatic
decreases in IPTG-inducible protein expression over the same time
frame.  Loss of pLysS alone should not cause reduced expression; in
fact the same pET-based plasmid in BL21(DE3) still gives monstrous
expression.
Do others observe the same phenomenon?  Is there a solution?  Are the 
BLR (RecA-) cells better?
Thanks,
Frank van de Loo.