2001 lumina ECM

I have a 2001 chevy lumina 3.1 V6 auto-trans w/OD, I just had a complete tune-up, new plugs, ignition wires, fuel filter, air filter, transmission fluid changed, had it for two days before the SERVICE ENGINE SOON light came on on a trip to Boston. When I got back home I called up the place that did the work and they looked at it and said that I was getting random misfires that was probably caused by the ECM. The service engine soon light has never came on in the past, I have 86,000 miles on it and take very good care of it. They said not to worry about it since it came on only when traveling at high speeds. I want to fix it without paying an arm and leg to do so. I just put many dollars into to it for the above.

What I am looking for is the following: the cost for a ECM, and the location of it under the hood. Advise to whether to try and do this myself or take it to a good mechanic who knows what he is doing.

make sure all your plug wires were put back on properly.and make sure they are not loose where thet plug into the coil packs.