1987 panasonic dx 5000...shimano 600 freewheel...6 speed

recently did the co canal. great trip, but...every afternoon the freewheel, i think, would start to slip. it felt like i was between gears and i would get that between gears slip. i tried adjusting shift lever of course and i tried different gears but no joy. slip would remain until i stopped for the day. the next morning everything would be fine. did this for 4 days in a row. every afternoon. it's fine now of course. weird .

maybe i should add that the temps in the afternoon were about 150 degrees

can't remember ever having freewheel problems before if in fact that's what it is. so this is a new one on me.

any advice would be appreciated

i did my due diligence on the search function. found some interesting stuff but nothing that addresses my issue directly

thks