Isuzu DYing Acceleration

I have a 94 isuzu Rodeo, manual shift. It has about 80,000 mi;es on it. I have had it since, 2001 until noiw 2007. When I bought it, it had 45,000 miles on it. In 6 years I have only had the clutch replaced and the pressure plate replaced. But lately it has been taking Suzy a while to accelerate. It's kind of dangerous when pulling out into traffic. The first 20-30 minutes of driving is O.K, but if I drive longer than that it takes longer and longer to accelerate. My wanna-be mechanic uncle took it for a ride and said it did fine with him (of course), but it doesn't happen all the time. I'ts very sporadic. He said it might be my air mass filter? Anybody have any advice for me?