Hi Richard,
I had this problem with the Futaba kit fitted to my Silver Lady. (I had asked Roundhouse to fit it for me so it was non-standard). I gave up on it and replaced it with Spektrum equipment and I haven't had a problem since. If you look some of the aircraft r/c forums you will discover that there is a problem with the maximum operating temperature of Futaba receivers. Spektrum work up to a higher temperature spec. It's not huge, but it's a significant difference.
Some of the cheaper 2.4GHz r/c kit is a knock-off of the better known brands so the Planet may be the same as Futaba.
Chris Bird, who posts on ths forum quite frequently, is a great fan of Giant Cod, which is another economy 2.4GHz manufacturer. I'm sure Chris will pop up and post something on this subject. He has a lot of experience with it whereas I only know about Futaba and Spektrum.
Steve