Karcher K3.86 doesn't seem to be able to sustain pressure. Keeps trying to pressurize then stops. Only keeps pressure while I spray. SOmestimes it totally shuts down, I guess when it overheats.

This is normal. The high-pressure car washer has certain dangers. If the water column of his spray gun is aimed at people, it will stab people. If the switch is turned on and spray water, then the pressure of the spray gun head will be sprayed around, which will accidentally hurt people. people, so it's designed this way.

replace the pressure regulator

Hi Stephen:
Don't know where you are, but in my location here in Canada, the local LORDCO parts store has a large selection of pressure washer accessories.
I'd suggest that you try to find a similar equipment supply retailer.
Cheers

sounds like a bypass valveproblem try backing off your control valve and see if it clears up. also make sure your hose and nozzle are free from debree.

Have you checked to ensure that there are no leaks from your hose or any of the connections?

Is there a water leak from the bottom ?
Is so, then there maybe a small crack in the seam of the plastic manifold pump housing or one of the connecting fittings. A New Manifold Complete about $61.00. You may purchase direct from Karcher, and replace if you are mechanically inclined.
If under 1 year old, and you can find a receipt, Karcher will replace the whole Pressure washer under warranty.

The motor doesn't pulsate when you are spraying and pulsattes when not spraying is normal otherwise the pressure will just keep building and burn out the pump. However, if you are not using the sprayer right away then the unit should be shut off until your ready to start again.
I hope the answer is helpful, please rate if the answer has helped in any way. Good Luck and have a great weekend.

Is there a water leaking from the bottom ? If so then there maybe is a small crack in the Pump Plastic Manifold Housing. A New Manifold Complete Replacement is approx. $61.03 from Karcher, if you are mechanically inclined.
If under 1 year old, and you can find a receipt,Karcher will replace the whole Pressure washer under warranty.

Not too much pressure, a bad gun.
Unfortunately you will have to replace the gun. About $65.00 Canadian

change the sparkplug