18 Psi Cut Evo6 1999

hii all

i just bought an evo6, the car does not have a boost controller and the boost hits 18psi.

there is no problem driving normaly, even if the boost guage shows 18psi, but the problem occcurs wene i give it to the max,
if i start from zero, and upto high RPM in first gear (almost 7000RPM) there is no problenm at all, but wene im on the 2nd gear, the car will cut at 6000rpm and on third gear it will be even lower it will cut at almost 5000, and again on 4th it will cut at 4500.

im not sure of what is the problem... i guess its fuel cut but not sure, does any1 know how i can check and find the problem..

and btw our country has only 94octan fuel, and this car was imported from japan.

another thing is that it is different at day time then night time, it dsnt happen at the same RPM every time.

the previous owner was not taking care of the car, im going to change the radiator and the engine fuel pipes soon. is there anything i have to check or change to figure out the problem.

might be your fuel. what are your engine modifications? maybe a possible fuel cut or may even be the Mass Airflow Sensor