Speed Control Cable problems of the 1999 Chevrolet Astro

Family and I were on hwy 441 traveling at 60 mph. I braked and went down to 50 mph as I began to make an exit off of this hwy. I let off brake and car sped up to 60 mph and the cruise control was never on. I was on exit ramp going 60. I was forced to put transmission into neutral. I coasted to a place off the roadwhile engine was "reving at high rpm's". I snapped the gas pedal and engine went faster. I reached down to pull on gas pedal and throttle cable broke. Vehicle was towed to a gm dealer the next morning. Their diagnosis was a dirty throttle body and stuck throttle cable. Never heard of an excessively dirty throttle body causing this type of problem, especially to a 3 yr. Old vehicle with 50,000 miles.