Hiya Graham, nice run over to Rathlin and back, I've been there many times, my first ever boat(dinghy) was given to me in Cushendall harbour (it was sunk) but it gave me the boating
bug when I lifted her. Great fish
and chip shop facing the harbour too. Google
cruisinginretirement.webs.com. and have a wee look. If your Volvo runs at normal revs then drops it sound like you may have under or over tightened a joint/banjo bolt which allows minute air bubbles to build up until the engine wont run properly, it happened to me and I've seen other engines do it and been really flummoxed until the owner pipes up that he changed a pipe a while ago. Many people tighten the fuel lines too tight thinking they are being prudent and all they need is hand tight and a wee 'nip' to get a good seal.