If its a FRPP replacement intake and nothing else changed I would chalk this up to an install error. Something not seated right, something not torqued right or something got messed up at some point during the process.
My advice is to pull it back apart and re-do it going real slow, making sure all the O-rings are good and the injectors are all seated and every COP is plugged in and make sure the intake is properly torqued in the right sequence. Also make sure all vac lines are hooked up and sensors are all plugged back in.
2000 Mustang GT Steeda build #48. Bolt ons, suspension, Novi 2000.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.