First and foremost, what are your RPM's and what are your water PSI numbers?
I would try a 25P Turbo prop. You should be running 72-75 MPH depending on your load. Also try raising and lowering your jack plate. Another thing to consider is weight distribution: keep the heavier items in your boat (spare props, heavy jigs, heavy soft plastics, rain gear, terminal tackle, tools, etc..) towards the back of your boat. Shallow water anchors and lots of gas will slow you down too.
I have 2 Power Poles, medium tourney load, 1 1/2 tanks of gas, a 250 SHO and I can do 73 MPH by myself. I'm also 325 pounds. With 2 guys (325 pounds each), medium tourney load, 2 full tanks, 2 Power Poles, I'm running 66-68 MPH... I need to mess with my setup a little bit. But that's also A LOT of weight. Each person's weight and setup is different so you'll get a broad range of answers.
However, you should definitely be hitting at least 70 MPH by yourself. If you're running a full tourney load, 2 full tanks, 2 shallow water anchors, and you're around 200 pounds, you should be running between 70-74 MPH. Something is definitely not right with your setup. Your prop might be too much or not enough, jack plate too high or too low, or else something's up with your motor.
"If people concentrated on the really important things in life, there'd be a shortage of fishing poles." - Doug Larson