My one-owner '89 K5 has 173,000 happy miles on its original TBI 350/700R4 drivetrain. Aside from my ongoing rusty body panel replacement program, for the past couple of months I have been experiencing a really irritating hesitation/rough idle problem which was so bad at one point that the truck was virtually undriveable, a radical departure from its normal silky-smooth, fast accelerating history. First, I did a full tuneup, adjusted the timing and replaced the EGR valve and fuel filter. A slight improvement. Then one spark plug was found to be cracked and was replaced. When she started acting badly again, some vacuum lines were replaced. This finally alleviated the rough idle. Again, she was okay for a couple of days until the exhaust pipe (I still have a single exhaust, can you believe it?) was found to be rusted from the cat back and was replaced. I figured the hesitation came from a backpressure problem and was surely solved now, but it still persists. I don't know what else I can possibly replace now. Any ideas?