Car Interruption/cut offs
 

Hi, I have a Chevrolet Venture model 2000. I started having a problem with the car out of a sudden when I was driving approximately 70mph. The car started to cough while giving normal gas, and the RPM meter would not go higher than 4x1000. After stopping the car several times and observing its behavior, it seems that it starts coughing when the temperature goes higher than 1/4. The normal temperature of the engine remains usually between the quarter and the half depending on the outside temperature and the driving speed and habit. The temperature seems normal, and the car works normal when I start it until the engine temperature approaches the 1/4 mark, then it starts interruption and coughing regardless how fast or slow I'd be driving it.

There are no lights on the dashboard that indicate any problem, and nothing seems unusual. The car was serviced approximately 1 month ago and new sparks, oil, oil filter, air filter were installed. It usually functions perfectly until this problem happened. I took it to the Chevrolet company to determine the cause of the problem to solve, but they said they will need to change items which are related to the gas and air intake system to determine the exact cause, and honestly, I don't feel that I need to replace a bunch of working parts just to determine the 1 failing one, as they try to sell parts and of course want to make money on replacement. I need to know what part(s) could have attributed to the problem so I would only order the parts related to change myself. Again to say, the car works perfectly when the engine temperature is cool, which draws me to think that when the automatic choke is open then the gas/air flow works good, but when the car engine heats up then the automatic choke will close and then it's not regulating the gas/air flow as it should, but that's only my theory. The first part I'm thinking to replace is the Mass Air Flow as I read it could cause such problem. But my question is, after describing the problem, what other parts than the MAF that could also attribute/cause this?
P.S. I already checked the electrical system and all seems good.

Thanks in advanced for the reply

Just an update
 

I did scan the car to find the problem, and I only got a code P0446 which refers to the EVAP, but when I called the dealer here they said the EVAP would not behave this way even if it would have a problem. In addition to that, if the EVAP has a problem, then it would display the Engine light but the light is not turned on, and it's not broken as it turns on normally when opening contact before starting the car with other lights.

Any ideas folks?!

Service Manual
 

Anyone knows where I can find a service manual for Chevrolet Venture 2000 to do repairs myself?!