When you put the new tank in did you make sure to vent the tank?? and was the hose and vent clear?? I had a spider nest in one of my vents and it caused all kinds of problems.
What kind of motor is it?? Like CB says ck all the lines for kinks or obstructions. Some time's iit's the little things.
Owning a boat is a great thing but it can be real fustrating at times and the spending of money will never stop!!!!!!!!!!!
There is a lot of nice mahcanic marine at
http://www.iboats.com/cgi-bin/ubb/ultimatebb.cgi
That might be able to help you. They helped me alot when I was repowering my boat and lookingfor a motor!! They told me everything to look for and it helped. Give them a try
Good Luck!!!!!!!!!!