87 starion won't accelerate during boost...will not go past 50mph either

I currently own a 1987 Mitsubishi starion. It's a 4speed auto and has a 2.6L inline 4
turbocharged motor. When I got it, ran great but now as soon as my car makes boost, the
gauges (rpm gauge and turbo gauge) goes up but the car doesn't accelerate at all. But
when I hit the gas soft enough that the turbo makes no boost, it accels normal...i can't go
faster than 50 bc it's an automatic and the trans always down shifts which makes the rpm
go up and the turbo gets boost...so my top speed is 50 (even that's a struggle) so my
cars a paper wight at this moment...people told me it's stuck in limp mode and it can be a
air leak but All my hoses are air tight and don't leak, and my turbo kit seems
good...anyone know what it can be? (My cars 100% stock no after market parts on
motor)