Macbook air not working unless plugged in?

I have a mid 2011 macbook air and it has had an issue of not working unless it was plugged in but mainly only working for a short about of time and then turning off suddenly and not turning back on unless pluged in or decreasing in power extremely and abnormally quickly. It happend a few times last year and twice i took it to a specialist specified by apple and twice i took it home with them having done nothing to it. They stated that there was nothing that the diognostics could see that was wrong with it. Eventually it fixed the problem itself and worked fine. just as long as i never ran it flat it worked fine. untill the other month i did that and now it will not work at all without being pluged into a power source. it has been plugged in all day whilst i have been using it and has not reach 100 per cent of power, it will not pass 98%. If the power cord accidentally falls out it turns of straight away, doesnt shut down just abruptly powers off. Never before has it been this bad. I have noticed it has also in this time it hasnt been working again its also been running noticably slower than usual. But that may just be me being parnoid. Im sick of taking it to get fixed only for them to tell me there is nothing wrong, because evidently there is. What options do i have? i just seriously need it fixed.