Engine over heating

I had a major tune up done in February on my 1985 Chevy C10 half ton. One of the things done was a radiator flush and change of coolant. According to the work order, a pressure check for leaks indicated there were none.

Six weeks later my heater began to behave irratically. I'd have heat for the first fifteen minutes or 2 miles (approx.) And then none. Having spent so much on the tune-up, I put off seeing to this--just no money left.

There was also an intermittent sound when I stepped on the accelerator, sort of a muted iron- filings- in-a-tin-can sound. I put having my under-carriage checked on my to-do list.

About 40 K out on a highway my oil light came up and I pulled over immediately. The oil, which I'd changed recently myself, was fine but my coolant reservoir had boiling coolant in it. (At the same time, the noise I'd attributed to my undercarriage became much louder.) About 5 liters of water got me where I was going and back home again without further problems.

The mechanic who'd done my tune up says the clutch on my radiator fan had gone. My Chev manual has nothing under troubleshooting or Fan clutches to back up the diagnosis.

I'd really appreciate any light you can shed on what happened!