someone help please check engine light on and off so cant have it checked runs great till you stop at a light?

I made the mistake of buying a reconstructed vehice for 1200.00. I put another $2000 into fixing damage under the car. Then I had to pay for an alternator, then 2 wks ago the water pump and belt went. Now the vacuum hose came off and we put that back on, and now the car stalls while we are driving it if the RPM's go below 10, and stalls at every light and corner. HELP.

Mechanics say that if it is not doing it when you bring it they can not narrow it down. Either they do not want to deal with it, or they dont care. I stalls then it starts, never turns off close to a garage or mechanic shop so that does not help.