Find attached a diagram of the 5210 fuel system. The numbers on the diagram are part numbers for the fuel and return line. Note they are 2 different sizes. I have a 99 5210. Here is what I would look at. When the fuel is low, look into the fuel tank and see, if there is junk at the bottom, which could be plugging up fuel going into the fuel line. The fuel line could have a small hole causing air into the line. As you may already know the fuel system is gravity feed ie there is no fuel/transfer pump.
If you decided to clean the fuel tank might as well replace the lines. If you do this message me and I will share my experience doing this. Not hard to do just takes awhile. The hardest part was getting the Philip screws out on the fuel tank straps.
Something else to try remove the fuel cap when starting.