You are the first person who has posted the exact same symptoms that i had been experiencing recently. I am 95% sure it it the MAF! I'll explain..
For the past couple of weeks my car has been running in a 'drivable' state, such that I could get it all the way up to 90mph by using say, 40% throttle through the gears. But any more than the 40% throttle and it stutters and looses masses of power. Back off slightly, (below the 40% mark) and it comes alive again. A nightmare driving up hills, as you said.
I didn't have any warning lights on at all. So my first thought was i'v had similar probs on a previous car from the lambda sensor. So changed that for new part. Made no difference at all.
Next, i tried a 'used' but genuine BOSCH MAF from eBay. Fitted it during my lunch break at work, then when i came to driving home problem was solved!! All the power back, right through the rev range, and under all throttle positions.
When i got home i thought, Right, I'll reset the ECU and throttle using the documented procedure, then I'm sorted. And that's when I started to get the warning light (injector symbol) on the dash.
Now my symptoms are as if though there is no MAF connected. It runs, revs, an pulls, but regularly stutters, and lunges forward when i back-off the throttle. (This is the response i was getting previously when driving it with the MAF unplugged, for a test drive).
This leads me to believe, either the ECU reset has buggered something up, or i'm unlucky with MAF's and this replacement only lasted a single trip!!
So where i am it the moment is; i'm going to check all plug connections tonight, reset the ECU again incase it just didn't work right first time round, and failing that buy another MAF sensor.
Crucially, The MAF did solve the problem for me, until i fiddled with the ECU.