Force 300-25 PTP link one-way RSSI degradation

I have a customer TDD PTP link using a pair of Force 300-25s over about 3.5miles, both on 4.6.2. RSSI is usually in the region of -55 both ways. However, just recently we are seeing one direction drop by 10dB or more, while the other remains stable. This was a roll-off over the course of an hour or so, remained so overnight, then slowly recovered the following morning. Frequency change does not fix it. With it affecting RSSI and not just SNR and with a roll-off rather than a step change, it would rule out interference.
Gut feel is that there is a temperature-related issue in one unit or the other, so we’re looking to change one or both units out. Anyone else had similar?