Hey nikku...wsbob here, a virtually silent member on bikeforums, but I'll second what cuda says about you probably not having flushed out the brifters enough yet. I myself went through that experience, watched a number of youtube vids. Unless the brifters appear seriously trashed, bent, broken, spindled and making some awful noises, they're probably just gummed up with the really good grease that shimano uses, but isn't so good after it starts to dry out as the years pass.
Alternative suggestion to using wd-40 for flushing, which I picked up from youtube vids, is use more of a cleaner...like that awesome orange brand stuff at the dollar store. That stuff cuts muck and grease like crazy. I rinsed well with water, I think. Dry with hair dryer. Follow up with maybe tri-flow.
There are actually two or three vid tutorials for disassembly and reassembly, but the procedure is kind of a nightmare. I wouldn't do it. At the time I was struggling with mine, I went to talk to the bike shop mechanics several times about it. They would tell me, 'those are throwaway shifters'. Didn't make sense to me. Shimano? No way. They make a good product designed to last a long time, but the eventually required clean and re-lube procedure is a major r&d breakdown to leave novice's. After I figured out how to get mine working again, I realized what the mechs were going to do with mine if I went for their story, was spend 15-20 min clean/lubing them up and then using them for themselves.