69WRX

Car is a 95 sti2, Fitted a manual boost controller about 3 months back. Car allways ran on optimax and octane booster, Boost was set to 18psi and its been fine upto now, no det, no spiking and car was flying.
The last week or so the boost has been fluctuating between 13psi and 18psi , but most of the time its getting to about 15psi then quickly drops off to around 13psi.
Any ideas on what can be the cause of this ?
Thanks in advance
andy