The decrease in power you’re experiencing after warming up could be due to a number of factors. It might be that the engine is not getting the correct fuel-air mixture, which can happen if there’s a problem with your fuel injectors or air intake system. Another possibility is that the spark plugs are worn and not firing properly.
Also, pay attention to your cooling system. If the engine is overheating, it can lose power. Keep an eye on your temperature gauge to see if it’s running hotter than usual.
You’re right to take this seriously, Mike. While it might just be a sign of age, it’s better to get these things checked out sooner rather than later. I would suggest taking your truck to a trusted mechanic for a thorough inspection.
Stay safe on the road, Mike!