Help,..Ram 5.2 Running Bad When Warm

My Old 98 Ram gots a new problem,...It's a 5.2 (318, 5 speed 4 wheel drive), standerd cab, Long bed,.....I can go about 4 miles till It runs Badly,...Backfiring, no power,No throttle response,......I push in the clutch and she runs fine, rpm's come right up,...let the clutch out and It just falls on Its face,...I checked the distributor pick up plate,...Speed sensor, Cap, Rotor, Wires and Coil have all been replaced,...Fuel Pressure is rock steady 48 psi,...No check engine light or codes,...I'm wondering about the O2 sensor (unlikely), or maybe the cam or crank sensors,...Any Idea's????