Exchange ServicePrincipleName SMTP disapearing

[MagAca]

Hi having an issue where intermittently the SMTP service principle name (both NETBIOS and FQDN) vanishes and all outbound external emails stop . I can put them back in using setspn.exe

 

setspn.exe -a smtp/server server

setspn.exe -a smtp/server.domain.co.uk server

 

but then after a random period of time they disappear and need re-entering.

 

Any ideas why these are being dropped ?

 

Its exchange 2010 on a 2008r2 server . Single server doing everything on exchange.