Same problem in my car in cold wet winter months. When you're moving very slowly the lightest touch of the brake pedal can cause a sudden stop.
It hasn't been too bad this winter but last winter it was bad enough that a light touch of the brake pedal at slow speeds was causing the rear left brake to lock up and skid on snow/ice.
On two occasions I stopped at traffic lights and was then unable to proceed forwards because the rear left drum had "locked" on. I had to reverse slightly to get it to let go which was very embarrassing with another car behind me....
The problem is with the rear drum brakes becoming excessively grabby although I'm not sure why, perhaps dampness getting into the shoe linings changing their frictional characteristics. I ended up giving the rear drums a quick clean and overhaul, including chamfering the ends of the shoe linings slightly so they can't dig in, and also lubricated the shoe backplates.
This seemed to help a lot but it has not entirely cured the tendency to be over sensitive in cold/wet conditions.
I've noticed that the drum itself is also ever so slightly out of round on the inside friction surfaces, this can also cause a drum brake to dig in slightly.
I did consider replacing the shoe linings - they have plenty of thickness left but I wondered if the linings were contaminated, however it doesn't seem that replacement shoes are available...(!)
I would have a look at your rear drums and give them a once over. To be honest the design of the rear drums in these cars are not that great. One issue is that the backing plate that the shoes ride on has a thick layer of enamel paint on it - when they get old the shoe frame wears though the paint to the steel, this causes the shoe to "jam" in the wear mark in the paint and prevents it moving freely to allow the two shoes to self-equalise.
This may also be a factor in the brake becoming "grabby". I was tempted to sand off the paint on the backing plate ears where the shoe frames contact so that the shoes would have a smooth surface to slide on but I didn't end up doing so.
I also found the handbrake self adjuster is not great - it has a tendency to try to over tighten itself so that when the handbrake cable is slack the shoes are just a whisker away from starting to engage, this may result in the brakes lightly rubbing in cold weather when the drum shrinks...
As someone who has largely only driven cars with discs brakes all round I have to say I'm not impressed!
Have a look at this posting I made a year ago on SpeakEV about overhauling the rear drums, including pictures:https://www.speakev.com/threads/p-ion-g ... st-1665961
Edit: Checking my speakev thread has reminded me that there is also a potential issue with the caliper slide pins on the front brakes that can cause the front brakes to stick on slightly, read the entire thread to find out about that one!
One slide pin is steel only, the other is steel with a rubber bush. The bush goes soft and mushy with age and comes out of the groove it's supposed to stay in, this causes the slide pin to bind in the caliper.
I replaced the rubber bushes on mine, only to have the same failure again 6 months later. In the end I replaced the pin that accepts the rubber bush with one that does not have a bush! The metal only version of the pin fits perfectly in both locations, and no more problems with the front brakes binding since I did that...
(I actually replaced all 4 slide pins with new non-bush types, as they were worn enough to "rattle" when the car was driving over rough surfaces...)