there are some great papers on long range PCR that I would look into. I would also double check your water, buffers, and pipettes (use PCR specific ones) as you could easily have DNase or other junk messing around with your experiments.
Also, try to check your DNA template on a gel again to see if its ok (no smearing etc).
One final thing, try a some new primer dilutions fresh from the freezer instead of your old one.
Please also post a gel picture as we can help more that way if we see whats going on :0
welcome to the forums