I have a fish in a similar state, but I figured it was due to
parasites or the like not a swim bladder problem. I was about to
remove the fish, raise the water temp and treat it with salt and maybe
praziquantel. All of my other fish are becoming more active, but this
fish behaves the way you're describing--laying on its side, swimming
away if I try to net it, and then lying on its side again. That seems
like a classic symptoms of parasites, doesn't it? My water is only
about 40 degrees right now. PH, etc. are all stellar.

I'd be interested in advice as well. If this is likely parasitic, does
it make sense to bring the salt level of the Pond up now or should I
wait until it's a bit warmer?