I own a '87 GMC Jimmy S15 and for the last few days it has been overheating and when I hit 35-40 mph it feels like it loses power and I can't accelerate past 45 mph. Someone told me it could be the water pump going out but then someone told me that water pumps don't just start to go out, they either work or they don't. Someone also told me that it could be a bad thermostat... but I have no idea and don't have a lot of money to spend on figuring it out. If anyone knows anything about my type of vehicle the information would be greatly appreciated.

First, check out the thermostat and then focus on mineral buildup or corrosion restricting radiator flow. If you keep driving the vehicle the way it is, you could warp the head and valves, blow the head gasket, and cost $2,000 or more to fix.