93 single turbo RX-7

Need some help here, my 3G seems to be acting up again. Just a few days ago I noticed when I was driving it, it sounded like it was miss firing, I could hear a tapping beat to the exhaust. I also noticed that on my boost guage the vac read only 12in at idle when normaly it is around 23in. I thought at first maybe I had a vac leak somewhere so I checked some of the vac hoses that i still had, seeing how I elimated almost all since going to a single turbo, and I checked the ones that I pluged, all fine. Cant explain why this is happening, perhaps it has somthing to do with my Power FC, but that has never given me trouble before and I havent touched any of the settings lately. Could it be somthing internal? Any suggestions would help, Thanks.