I don't run my little 800/900 regularly but it works okay after having for several years.
How are you measuring output? Mine will put out 900-1000 watts regularly.
I have found that it is a little erratic. I have the output voltage at 130+ unloaded in order to have 115-120 under load. It is adjusted by an easily accessible screw.
If you read the reviews on the HF website they are mostly positive. It seems that most people have the opposite experience you are having. Many are bad to begin with but once you get a good one they run. I had to return the first one I got because it had been dropped and damaged before placing in a flawless box.
My only real complaint is that it is hard to start and has always been hard to start. It takes 10-15 or more pulls when cold. I've changed plugs and made adjustments without improvement on the starting.
