2003 Toyota Avensis engine running, sluggish, no power

hi, ive had my evensis for a couple of years and was very happy. a few weeks ago the engine felt like it was missing and lossing power. i put some injecter cleaner in and it fixed it for a while.. but it soon came back.. i took it to my local garage and they said it was probabley the air flow meter.. they fitted one and it hasnt fixed it and they are stumped as nothing is showing up on the diagnostic.. my next step is going to be a toyota dealer.. maybe new injecters.. £1600.. can any one help before i go down that road .. thank mark