Intermittant fault causing poor running on Toyota Corolla

HI. I have a 1998 1.6 toyota corolla gs 4A-FE engine. Manual gearbox with 49,000mile on the clock. Has been serviced regular and had timing belt changed recentley. Its due a service now though.
sometimes when driving the car it feels very sluggish and flat. It doesnt feel very responsive. There also seems to be a bit of judder or hesitantsy. Then other times I start her up the engine feels very responsive with crisp acceleration and no hesitantsy. There is no paticular time this happens although the car will not run well when it has aready been started and runs flat if you know what I mean. I can go back to it later on, start her up and she runs crisp again. Very annoying. Did briefly have it in at a toyota garage when I felt it running poorly and they gave it a quick look over and couldnt find any problem. They couldnt even notice the fault on the road test but they dont drive the car every day, I do. Please please can you help. Cheers Berni.