td42 turbo gq boost issue!

Hey guys just a question thats been doing my head in for a few weeks now i have a 93 for maverick td42 with aftermarket turbo, the original dts turbo blew so i got a ebay turbo to do me for now, everything was fine running 12 psi problem now is boost goes up to around 10 then drops to around 7-8 we thought the obvious with a boost leak so checked all piping and clamps adjusted the waste gate everything seemed ok tightened everything went for a drive still same issue so we turned the boost tap up to compensate seems to be boosting ok to 10 psi then struggles a little to make the 15, any help at all will be appreciated as im stuffed lol...