can anyone help rx7 tt prob

[mr d]

hi please can anyone help me i have a 1992 rx7twin turbo has a serious fault when you accelerate above 4000 rpm it drops its boost have changed every thing on this car to try and cure but no luck have tested 12volts to control this shows constant 2volts then when you accelerate it creeps up to 8 volts enough to open waste gate and drop the boost this should not happen should be 12 volts or nothing cannot find were 2 volts is coming from or why it creeps up this car has only blitz fillters and a hks dump valve fitted no other mods would be grateful if anyone can shed some light on this prblem thanks .