1994 Chevy S-10 very erratic engine behaviour

I have a 1994 chevys10 and in the last few weeks it has been acting up. Out of the blue it will idle strangely, first stumbling and then reving very fast, then stumble again and then eventually stall. Once it stalls it wont start again. I go back to it several hours later and try to start it and it runs fine. Other times it will run fine I will park it go back an hour or two later and it will not start. It turns over fine just will not start. I took it to a local shoip and they werent able to help saying that if its running they cant help me. Any suggestions would be greatly appreciated. Thnks