Power reduction

I have a 1994 2WD 4 cylinder toyota truck with 159000miles and have recently experienced a severe power reduction. It happened quite suddenly, and seems like it is a timing problem. The truck starts and will idle rough at 1200rpm, but when it warms up and reduces the idling to 500 rpm it will choke and die. I took it to a mechanic and he said that it had a worn timing chain that needed to be replaced, but I am pretty sure that he did not open the head and take a look at the chain. I did open the head cover and looked at the chain and it appeared fine (although I don't have experience in diagnosing a worn chain, it was tensioned and looked new and I cannot imagine that it could "slip"). I have taken the truck in for a tune up before and had the timing reset, but it has never been this bad. I now know a bit more about the truck and can fix most simple things myself, so I am planning on changing the distributor cap and rotor and the wires, reset the timing, and see how it goes, but I am afraid that it is more than a timing problem. Any suggestions?