Frequent crashes with iMac 11.3

These memory chips are of the 11.3 iMac (27 ", mid-2010) recognized if one leaves the old RAM modules in situ.
Starting running downright and operation places also, but there are often crashes. For example, when the synchronization between iPhone 5 and iTunes. Once you remove these blocks, everything works again as usual but just slow.
The iMac will not start if only these two RAM modules are used.