The brake lights are on but no noticeable problem?

I've had this car for about 5 months now and I bought at a used car dealership. I haven't had any problems with it until recently when the brake lights turned on. At first, the brake light would turn on but turn off when you restarted the car. Nothing noticeable was wrong so I just shrugged it off. It did that every once in a while but one day it suddenly wouldn't turn off. On the day it didn't turn off, as soon as i took my foot off the gas, the car would slow way down. It had never done that before so I immediately took it to the shop to get it checked. The man said he didn't notice any problems and all the equipment looked brand new. When I left the shop, it was working perfectly fine, no lights, no slowing. About two weeks later (now) the brake light is on again but there is no noticeable problems. The car shop couldn't check the electrical part (they didnt do hybrids) so I have to find a shop that does. Does anyone know the problem and how much it will cost me?