Lost power high idle

Hi all this will sound strange but bare with me i think my vrs has lost power it makes abit of a farting noise under hard acceleration maybe exhaust down pipe haven't had chance to look also the idlemis a little strange wen warm it idles at 800 but of late it decides it prefers 1000/1200 and stick this usually occurs whilst slowing to a stop a quick tap on the gas and drops bk to 800 no problems any ideas? Hope this isn't the start of a run of faults :( cheers