The cruise control problem may be related to the boost problem. I would figure out the boost problem first, and then worry about the cruise control. The boost problem could be caused by a number if things. Most common is the y-pipe coupler. After that, it could be any number of hoses, solenoids, pills or actuators. Here's a good place to start...
http://www.autosportracetech.com/RX-7/rx7stuff.htm