Ip phone 7942 falls off during long conversations, what could be the problem?

Question

Cisco

Good day. There are 3 Ip cisco 7942 phones on 2 different fields. During long conversations (1.5 hours+), 2 out of 3 phones tend to disconnect (loss of connection with the server and attempts to re-register). (2 on one, 1 on the other). There are many more similar phones on the network, but (as I understand) there are problems only on these.
Data transmission channel: satellite channel (satellite communication terminals are tuned to a sufficient signal level (16dBm with a mirror of 1.8 m)), while the rest of the local network services work properly, telephony on sip (2 ap100b voice gateways) works without problems.
Network: distributed LAN.
tsisko-server one and the same.
We tried to register on another server, change the PSU, a separate stabilizer and uninterruptible power supply for network equipment (there were suspicions of a power drawdown due to the start of the engines of a large drilling rig). The problem also remains open. Since I am not an expert in the field of ip telephony, I will listen to your comments with great pleasure. Thanks in advance.