B2300 No Power/hesitation

I have a 1997 mazda b2300 that has no power and has hesitation issues off and on. I have also noticed that there is at times decreased back pressure. It's a manual and when letting off the gas...it doesn't always slow me down like it should. Have replaced the plugs and wires tried fuel injector cleaner. Tried new fuel filter. No codes are showing up. Any suggestions?