2001 4.8L Loss of power
 

My 2001 Silverado 4.8 V8 with 189,000 miles seems to be losing power around the 75-80 mph area (just so happens to be the Texas Highway speed limit area), just after switching into 4th gear. It has a hard time staying in 4th gear, I'm getting tired of it downshifting into 3rd, even more when pulling a trailer.. I was hoping you guys could give me a list of things to upgrade in the engine to keep it in 4th. I've already added an exhaust, cold air intake and TBS ... I know new spark plugs and wires are coming up (any suggestions?) What else can I do ?? I don't want to buy a new truck if this one keeps running good

Do people agree with this?