Misfire and loss of power at between 2800 and 4000 RPM

First I need to start off by saying that NO Check Engine codes are thrown by this issue.

Car is a 2007 Mazda CX-7 turbo, non-awd.

What’s happening:
This can be broken down into 2 section really, a cold engine and a hot:

Cold:

When first starting the car and it’s cold out (around 5 degrees C, or less) the car will usually not idle well and if left alone will sometimes stall once or twice. The first 5-10 minutes of driving will usually have a lot of misfiring and overall running crappy even at RPM lower than 2800.

Hot:

After the car warms up, it will generally run OK at idle and normally up to around 2800 RPM. At 2800 RPM it will misfire and jerk and buck if kept under constant throttle. I usually have to back off the throttle and get back on it slowly to avoid hitting that RPM range. This of course makes it quite difficult to merge onto freeways. To get past this issue, what I often do is build up some speed, then put the transmission into manual mode and drop it down to 2nd gear. Doing this will bump the RPMs up to over 4000 (if it does breach 4000 it will not accelerate), and once it is above 4000, it runs smooth and will accelerate normally. Dropping at any time below that level will get me stuck in the power loss and misfire issue again. This issue is further pronounced when going uphill.

What I have done to attempt to fix this:

• I’ve replaced the plugs and coil packs, no change in this problem
• Cleaned MAF, no change
• Replaced MAF, no change
• tested and replaced O2 sensors, no change
• did prelim check for vacuum leaks using brake clean, none found

Next steps?

My next thought was that it may be an issue with the fuel system, but the fact that if I can get it over 4000 RPM that is runs perfectly seems to eliminate that possibility, or am I mistaken? Thoughts?