I had the same type thing happen last year when I first bought my 55. It would drive great for about 15 minutes and then start sputtering. When I would come to a stop it would cough and sputter then die like it was out of gas or vapor locked. I could floor the gas peddle and turn it over for 10 seconds or more and it would fire back up. I put on a clear gas filter and could see it was starving for gas when it was doing this.
I ended up changing the tea pot carb for a Holley 390 and dropping the electric fuel pump for an FE pump. Put in new gas and it ran great after. I had also dropped the tank and found it was sealed previously.
The final variable was in draining the gas from the tank (winter blend bought in March) and replacing it with fresh gas in about May (summer blend). I was told later that it may have been the winter blend in combination with the electric pump causing a void in the fuel delivery to the carb. I do not know about this for sure, maybe someone else has an opinion on fuel quality.
