hydraulic lifter problems

iam running a comp cams hydraulic flat tappet cam in my 360 i purchased the cam kit with the high engery lifters iam running the mopar performence 1.6 roller lifters the motor would not rev over 5500 rpms i called comp they recamended the pro magum lifter they want 2" to 4" pre load with the engine hot hard to set perfect with intake on but i think iam closer to 2" now the engine breakes down at 6100 rpms iam running the 995 springs should be enough spring has enyone run into this problem with this combo