Hi I am currently restoring a 1999 Subaru Impreza uk turbo 2000 it had a very rough idle issue and would rev up like you would expect a sports motor bike to and after doing a substantial amount to the engine including the following,

service, new plugs, new leads, injectors cleaned, turbo rebuild many parts removed cleaned and painted all vacuum hoses replaced and I replaced the air temp censor I believe that’s what it’s called it’s the sensor on the side of the throttle body the idle issue seemed to be solved but after letting it run for maybe 20 min the rough idle started again has anyone got any ideas what I should try next. 

was it stood for a while? could be dodgy fuel? MAF sensor could be an issue as well. Starter for 10 might also want to do an ecu reset to restor it to default.