The shutter/mirror/aperture stop-down mechanism is fairly complicated and involves multiple mechanical interlocks. From your symptoms, it sounds like a sticky mirror bumper foam (at top of mirror box at leading edge of focus screen). Test with a toothpick or splinter of wood. If it is missing or anything other than springy it needs to be replaced. There might be other issues as well since you say the position of the meter switch changes the behavior.
You can do the mirror bumper (and foam light seals) yourself. I recommend the excellent kits from Jon Goodman ($10 includes shipping...
JGood21967@aol.com). Having said that, I will give the standard advice from the sage users of this forum. Send it to Eric Hendrickson. Eric will do a full service that will put your camera in as close to new condition as possible and do so for a very modest amount. Yes, the price will be about the same as what you paid for the camera, but when it is done, it will be good for the next decade or two of regular use.
Eric's Web Site: pentaxs.com
Estimates are free. You pay the postage to send. Eric pays the postage to return. He does this even if you say no to the repair. Great guy, that Eric.
Steve