as stated above id flog your 40mhz on ebay and buy a dx5e or similar for about £60 from a model shop.. If not..

In the first turbulence we had loads of interfearance problems.. the best solution was to cut the ariel off. put a spade connector on what was left (about an inch) then co-axial cable going from the r/x in the robot to the boat ariel on the outside of the robot (same as youve got) it solved most problems.. i also wraped the r/x and upto the co-axial in tin foil and put the r/x in a plastic box.

also- your power cables (red and black from battery to speedo etc) im told if you wind those together in a drill for example that can help reduce noise.. aswell as keeping r/x wires away from main power cables.. supressing your motors etc..

But after all those mods.. id still suggest a 2.4kit