Engine Misfire Issues on 99 Camry,300, 301, 303 and 305 code

I am getting a 300, 301, 303 and 305 engine codes. Check Engine Light blinks. There is a Foul smell from the exhaust and at certain speeds the car shakes. Changed all sparks plugs, coils and cables with new ones from Toyota. However, problem is still there. What's puzzling is that the problem (and Engine light) seems to stay on for 2 weeks, go away for 1 week and then come back.

99 Camry
6 Cylinders
170,000 miles

Please help.. already have spent a lot of money and problem is still there.
The P0300 (random cyl. misfire), P301 (cyl. 1 misfire), P0303 (cyl. 3 misfire), and P0305 (cyl. 5 misfire) tell me that all the cylinders on the left side are misfiring. The 'foul' smell is caused by the catalytic converter 'burning up' the unburned fuel because these cylinders are not firing like they should. This will destroy the catalytic converter very quickly. The only thing that immediately comes to mind for this scenario would be, possibly, timing belt has 'jumped'? You need to get immediate attention to this problem. If overfueling, engine damage is possible. If the belt breaks, real engine damage is possible.

SW Washington One other possibility is that there might be a vacuum leak that is common to these three cylinders?
A large enough vacuum leak, will cause what is called a lean misfire; not enough fuel to support complete combustion.
This is a double edged sword; raw fuel from the misfire will enter the exhaust system doing exactly as Loren said to the converter. But when there is a vacuum leak and a misfire; there is a lot of leftover oxygen that hasn’t been burnt. This oxygen is dumped into the exhaust system. The O2 sensor reports this excessive oxygen to the computer; in turn the computer will command the injectors to add more fuel in an attempt to compensate for the excess oxygen.
In a sense; the engine now is a big fuel pump, sucking fuel in and spitting it out the exhaust or better said into the converter.