How to get trasit van radio working (FM)?

My dad has a ford transit I don't know what make but the dam thing had a caset player, so it's fairly old. If you switch the radio on you will get all normal UK stations, Radio 1 for example. But you only get theses stations for 3minuets at max maybe we got an hour once, never happened again.
Here is the strange part, there's FM channels and AM channels and AM works 100% with no issues at all, is there anyway to fix this? Had this happeneded to any else?

I would ditch the radio and aerial and buy new ones, Second hand radio from Ebay New aerial from Ebay

Andy C