2002 Chevy Cavalier engine sputters after 4500 rpm

my car is at 76000 kilometers and idles great, runs smooth but when I was doing highway driving in BC (many mountains) and had to do a lot of passing. I notice that mainly on hills my car would accelerate great untill I hit 4500 rpm it would then start to jerk and power loss would be felt, the check engine light even started flashing when I kept pushing it and only stopped after I took my foot off the gas. I have new plugs, oil change, a clean K&N cold air intake system, a new fuel filter, and I cleaned my injectors not to long ago. My car was sputtering 4 months ago but I did all of the above and solved the problem. I did have platinum plugs in it after the tune up for a month which made the sputter untill I changed them for iridium plugs which is what the ecotec engine needs, I then used a fuel injector cleaner after that just in case. my mechanic is telling me it could be the spark plug track or my injectors and he isnt sure. Please help, what could it be.

Matt