idle automatically adjust down

That sounds like it is working right.

Mike

Have the throttle body and idle air control valve been cleaned? Normally that is more of a problem at idle, especially cold idle, but may as well cover the basics.

Otherwise, there are two common problems that can give those symptoms. The fuel pump relay comes to mind first - the solder connections that mate the relays to the circuit board inside the module crack and become intermittent. Some people pull the cover off and resolder the connections, others pony up the $40 US or so for a new one. The relay is usually worse when the car is warm than when it is cold, but mostly it does what it wants.

The "power stage" that drives the ignition coil is also famous for intermittents when hot, especially the sagging type. The tachometer will usually drop before the engine speed does, even dropping to zero before the engine dies. The problem is generally cured by removing the device from the body, cleaning the mounting surface and even applying heat sink compound (available at electronics stores or computer shops) before snugly remounting.

But if it isn't a coincidence that it stops acting up when you whack the hose to the air filter, the Mass Air Flow sensor, also called an Air Mass Meter or AMM, could be the culprit. They definitely can become intermittent and I don't know if they will set the "check engine" light in a '92. Next time, instead of disturbing the hose, carefully apply upward pressure to the flat connector on the AMM (that device in the duct right after the air filter). If the engine surges when you lift, you may get away with removing and carefully reseating the connector (it worked for me!) If tweaking the connector doesn't help, try rapping on the AMM itself. They are expensive to replace but a bad one won't do you any good at all. They are also available from wrecking yards.

Mike