This problem is usually caused by the VCO unlocking, although it can be caused by the internal frequency having a bad connection frequently it is a grounding issue with it or the VCO inside the cover on the side where the speaker is located you will find 2 metal boxes check all connections to them and the grounding screws. Hint there are also grounding screws located inside these metal boxes keep them tight by applying a little glyptol or fingernail polish to them once tightened. I have also had to solder ground wires to both cans to help keep down on these issues particularly if the radio is subjected to a lot of vibration.

Crystals don't like cold weather, crystals is what controls your freq. it could be a bad soilder or bad crystal

This sounds like a classic RF feedback issue. Check and adjust as required the antenna's length for lowest SWR for the frequencies used. A mismatch of SWR greater than 2.0:1 (or "two to one") is going to cause problems for the transmitter - the higher the mismatch the more likely it will result in failure of the transmitter. The SWR is an indication of power that is reflected BACK to the radio from the antenna. A low SWR (1.1:1 or "one point one to one" is ideal) means all or nearly all the RF power is being radiated out of the antenna. A 3:1 SWR indicates a very large amount of the transmitter's power is not radiated out of the antenna, but is being reflect back into the radio. This causes all kinds of unexpected results - one of which is final transistor failure if allowed to continue unchecked.

Good luck!