1) unplug the map connector and see if does the exact same thing. if so then the wires to the map may be hosed up, or the input pin of the ecu is bad.
2) not 100% sure where the tach gets the rpm but if it's from the ecu then maybe it's really redlining where it's supposed to and u just need a different guage for that ecu. get an obd scanner and read the true rpm's. while u have the odb see if you are getting readings from the map.