Cars House

I just bought a 1992 325. I test drove it for a while and before the car was warmed up, it would sputter, backfire if you tried giving it some gas, and then eventually stall. It did it twice and then once it was warm it did not do it at all anymore. The guy said he took it to a shop or a dealer(i forget) and the mechanic said that it was b/c of a performance chip that was put in it. The performance chip was supposedly not for a car with no modifications. I have driven the car for about 2 weeks and it only stalls once or twice before it warms up and then it is fine.It also has a rough idle. Does it sound like this guy is bullshitting me. I took apart the ECU and it belongs to a 91 525i. But i couldn't tell the chip in it was stock or not. Could anyone help me out here?