This plasmid is quite old and has been stored at -20c since 2008. When I miniprep it from liquid cultures of about 5ml, it always runs slower on a gel, including the digested fragments. I also find I can't generate retrovirus using it whilst I can with the original stock. Does anyone have any ideas as to why this might be? Many thanks!

Please see attached image - numbers indicate different minipreps, OG refers to the original plasmid, and Un to undigested.