Hi, I'm relatively new to road biking. I have an '80s era Peugeot road bike. I've replaced some of the components, most notably the freewheel. It has the older gear shift levers on the down tube and I believe the deraillers are '80s era too. Lately, I've been finding that when I'm going up steep hills (in the lowest gear) and exerting a lot of pressure on the pedals, the chain will shift from the largest rear cog to the second largest by itself. Sometimes, it will go from the largest to the second largest and then to the third largest. This is very annoying when I'm trying to make it up a hill. If any of you out there might know what the problem is and how to fix it it would be greatly appreciated. Thanks.