Bill is onto somthing, if this is from a motor swap did you get the overbost protection switch and plumbing from the donor car? Try temporarly running a preasure gauge T-ed into the boost sensing line that runs to the ALDA on the IP.
You should get From 10 to 14 psi at full throttle under load at 4000 rpm. You can adjust the wastegate and ALDA to max factory specs and really improve things. Also are you starting in first gear?

Bob D.