I have had this issue with almost every 5 speed I have driven. You need to put the car it 1st before going to reverse. You don't need to get the car moving in first, just shift to first, then without releasing the clutch, immediately go to reverse. Be sure you have that clutch clamped down hard, reverse can be a real PITA on a lot of older 5 speeds. You will get use to being able to tell when you are or are not fully in the reverse gear. This can be tricky because the car can be partially in reverse, then when you start rolling you hear a light bang and the shifter spits out of reverse. The other thing that often happens is that it just won't sync reverse and either won't go in at all or grinds for a moment on gear entry even without releasing the clutch. As far as I can tell these nuances are common to nearly all manual 5 speed transmission designs. Even my new 2009 WRX occationally pops out of reverse or gives a light buzz on entering reverse if I dont hit 1st gear first. I almost always go to first before reverse, it just comes as second nature. BTW cold weather can make the whole reverse gear thing worse.
