Based on what you've described, I'm thinking the motor may have the original nylon teeth on the timing gear for the camshaft, and they have started to break off, causing erratic timing. One way to check is to drain the oil, and look for small particles of plastic in them. Another is to pop the distributor cap, put a socket on the center crank pulley bolt, and rotate the engine in reverse. If you notice a good amount of time for the rotor on the distributor to turn, (it's hard to say how much because it's been awhile since I did it) it's a sign the nylon has started to break off. The only true way to tell is to pull the oil pan and look for pieces in the pan and the oil pickup tube, or the timing cover and look at the gear and see how much slop is in it. (might as well swap it and the chain while in there). If there are teeth missing, make sure you clean the oil pump pickup tube, as it's most likely packed with bits of the nylon teeth.

Don't want to scare you, but if the teeth break off enough to cause the chain to skip a tooth, that's called "jumping time". Which means the valvetrain is no longer in time with the pistons. And since a 318 is a interference motor, when this happens a valve hits the piston and at a minimum causes a bent valve, at worst, a busted valve in the piston.