Maybe someone can help direct me with my troubles.

Symptoms:
1. Rough idling
2. While cruising at about 45 mph, I can feel the engine hesitate (behaves sluggishly)
3. Fuel economy dropped

Current condition:
'05 Limited, manual drive
29,300 miles
Last oil, oil filter, and air filter change conducted at 25,000 miles
Environment: Hot and Humid; 90 deg F and 75% humidity

Corrective actions:
1. Tire pressure corrected
2. Verified air filter was clean and clear
3. Burned all gasoline and replaced with Shell premium (had been using premium, action taken to eliminate the bad gas theory)
4. Reset throttle wire
5. Cleaned fuel system and fuel throttle body
6. Said a prayer for enlightenment

Mechanic doesn't know what the trouble may be. Any advice before I lose my wallet at the dealership?
Also, do you recommend extending the powertrain warranty for $150? What is it really worth? It will only cover the transmission and internal mechanisms of engine (i.e. piston, but not seals)