Toyota Repair: 95 previa, modulator valve, driveability problem

Question
QUESTION: I am rewriting this question because the last time the problem went away and now is back.  Thought it might have been the gas but problem came back so i drained the fule system. As I said before i did a full tune up cap rotor plugs wire pvc fuel filter the works.  The problem is when  I start out wether it cold or hot.  when i accelerate from first to second or even in first gear the car starts knocking. If I give it a very slow amount of gas and slowly nerse it up in rpms the engine will run ok, but when i deeccelerate and stop and try to get up to speed again it bogs down and I have to slowly , i mean very slowly add some gass, it might run again in an upper speed.  did a system check and came up with error # po125, po130, po133, po135 and po 401 any ideas. could it be the erg valve????
ANSWER: The first three codes indicate a problem with the O2 sensor in front of the catalityc converter so I would recommend replacing it, the 401 code is an egr malfunction code but it indicates an insufficient egr gas flow which would not cause a driveability problem, in most cases this code is set because either the egr modulator valve is leaking or there is a blocked vacuum line at the throttle body or a blocked passage in the intake manifold that restricts the egr gas flow.

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QUESTION: cleaned out the throttle body as directed in the manual that I have.  The interesting thing is that I found when the car is stopped i can hit the gas and increases and decreases with out a problem. I started on a hill, put it in nuetral and started going down the hill with the gas sligtly pushed down so the van was reving. As the spedometer hit around 20 mph, " still in nuetral" , it started bogging down and continued that way untill I came to a complete stop and the bogging stopped and it accelerated freely. When I drive it now it starts to run crappy around 20 mphs and will not let me go above 40 because of the bogging. It runs crappy after 20 and the only way to reach 40 is to slowly add gas.  Would the speed control be faulty and is it easy to change.  

Answer
I would start to consider a possible catalityc converter problem which may be clogged and also restricted the exhaust system, if you have a vacuum gauge connect it to direct intake manifold vacuum, at idle it should have 18-20"hg(inches of mercury)on the gauge, a deviation of more than 5"hg would indicate a problem.