1995 Nissan Hardbody 4 cyld 5 speed cuts out

I just bought this truck. At 3500 rpms it cuts out, it seems to run fine up to that point. I did all the usual tune up proticols, plugs wires cap rotor fuel and air filters. As well as put some fuel injector cleaner in the tank. Now it runs a little rougher and in 5th gear it doesn't wait for 3500 rpms it begins cutting out at 2800. I can push it up to 3200 rpms in 5th gear but no further. Also the speedometer has stoped working. The check engine light comes on periodicaly, when I took it to the mechanic he said that the computer would not spit out any codes. He said if he were guessing in the dark he would start with the throadel body sensor. Any and all help would be apprietiated
thanks