98 MPV misses under load when warm

We bought a 98 4WD MPV from a local Mazda dealer in a certain Albertan city nearly 18 months ago and have had little but trouble with it. It was in every few weeks in the first year. About 3 months after the purchase we had the fan clutch replaced by the dealer, who installed one that was very noisy, it sounded like a truck! He said it was too big for our car but he bracketed it in and it would "do fine". It ran constantly while the engine was turning. Since then we have had one major problem that renders the car completely unreliable, and yet none of the Mazda dealers here has been able to diagnose the problem, nor fix it, supposedly because it is intermittent, and also because no panel warning lights show up. Initially a warning light did show up but after taking it to a nonMazda mechanic who did something with the computer diagnostics, it hasn't come back on since. One Mazda dealer told us that that light may have meant it needed an fuel-filter change, which we had done at that time. It was filthy even though the dealer we bought it from had checked it off as having been changed in the recent servicing they gave it.

This huge problem is that when the weather is warm (15 degrees C) and the engine has run for a while, it begins to miss, especially under load. Needless to say we bought the car when it was cold, in spring and the problem was not evident; it may have been there but didn't show up because of the cold temperatures. We requested all previous maintenance documentation from the dealer we bought it from since they had serviced it for the only previous owner, who by the way, worked for a Mazda dealership here in town. They refused on the grounds that the information contained the previous owner's address etc, and so we said "cut it off" but they said no anyway. Can we insist, with the law on our side, to gain access to the records?

More about the symptoms of this problem:
-engine misses under load when engine & air temp warmish, as above
-going up long shallow inclines car may not make it
-problem may disappear briefly if engine turned off for few minutes
-at idle miss can be felt
-in neutral, pedal flat to floor, engine will only reach 2000-3000rpm

The distributor, waterpump and the exhaust oxygen-sensor were replaced but did not affect the problem.

Initially we thought it might be an electric problem caused by thermal expansion in a broken wire or a dry joint in a circuitboard, but it could just as well be fuel-related. We have checked for a PCV valve as per recent thread, but on our car there is a rubber hose clipped to the middle of the back of the inlet manifold which runs to a stamped-steel diaphragm-type valve under the right-rear of the manifold; from the front of this valve runs a short steel pipe to which a fuel-type hose runs off to the side & down behind the engine; I assume this is our PCV (positive cranckase ventilation) valve system. The hose appears to be fine, but the diaphragm may have split as such items can do.

We used it to go to the Rockies last weekend, and had to stop several times on uphill grades, barely making it up one of them at all.

We don't know what to do. We refuse to sell it to a private owner as it is, and of course the Mazda dealers know it has a problem and won't touch it. Maybe we ought to trade it on an Estima (=Previa)?

Can anyone shed some light on this problem?