The fact that your radio is 4-5 years old is not so significant. What's really important is how much use has it had. If you've been flying a couple times a week for 4 years, then it would definitely be good to invest in at least a new flight pack, if not an entirely new system. If you've only flown 10 times per year, then the thing is just getting broken in. As has already been mentioned, check your battery capacity. If the plane and radio are in good shape, you may be able to sell the whole setup, minus the engine, to a beginner and use the money for a newer radio.