loss of power

I have a 95 villager that losses power intermitently. If I shut it off and restart it , it is fine for a while. Sometimes I have to disconnect the battery and hit the key several times to "reboot" the computer, and then it will be OK. A mechanic told me it was the ox. sensor, but that didn' t do it. Then they said it was the knock sensor. Both work temporarily , but then it s back to losing power at highway speed, or even driving locally. Any ideas, I don't want to keep having to put money into parts that don't fix it, HELP

Do you get a check engine light when this happens?
How many miles on engine?
WHen was last tune-up?
Have you checked for any codes?
Have you seen any change in gas mileage?

The car has almost 200,000 miles on it , I get a light but then it will go out. The code I first got pointed to the ox. sensor, which I changed and the problem went away. I then took it to the mech who said a code turned up for the knock sensor. It has not been tuned up recently. Gas
mileage has gone down though I dont really keep track of it, I just judge by how often I need gas .

I would start with checking the fuel psi with a T fitting in the fuel line and run the gauge inside of van and drive and see what the psi is when problem occurs. If ok,I would suspect the distributor/cam sensor is going bad.