problems with some ptp

Hi, I have a problem with some ptp.
I use 433AH or 411AH with one R52hn but if I use it in single polarization i have often problems with bandwitch test.
Sometimes I have 10 MB in download and 30 MB n upload but sometimes I can have like 70MB in download and upload.
I really understand why..
Someone can I help me?
What is the best configuration? Where is the problem? R52hn are good only in double polarization?

thank you

but miktrotik’s site writes about R52hn:


Support for up to 2x2 MIMO with spatial multiplexing