Sounds like you run through pully belts.
It maybe belt slippage,
or blockage in your inline, filters. 

Where are the inline filters 

could be hair or somethign stuck in your regulator or pump or line or what ccworks seaid 

What TM are you using, maybe some one can help. As you seem to know nothing about your TM maintenances. 

If the machine has a Dema solenoid or other type it could be opening up and draining water into your waste tank. My pressure drops some when mine opens. You might try and turn the temperature control up and down on your machine and watch the pressure after the machine gets hot. When you turn the dial down below whatever temp the machine has actually reached that solenoid should open and the pressure will drop some then go back up when you turn the temp dial back up. I can tell if my Dema is working correctly by doing that. It's possible yours could be opening and staying open. What are you setting the temperature at when this happens?

Another possibility is the pressure gauge is bad. Check to see if your pressure gauge is going all the way down to zero when the machine is off. It could be giving a false reading. Can you tell for sure that there is actually a drop in pressure at the tool? I'm guessing you're doing hard surfaces at that kind of pressure. 

Am using the Black Stallion by Chem Tex 

If you dont know what an inline filter is you need to find someone local to teach you about the machine and maintenance of it. 

I agree with ccworks. I'd look at the belt and possible slippage. Good luck 

Check & clean filter in the water box 

1200 is high for that machine I would say,what is that a 15 hp I had a 14/33 I wouldn’t go over 900 psi 

I agree with Randy.

Disconnect the bypass/Dema solenoid run machine and check. It could also be that your machine was never designed to go that high and is calibrated/adjusted to bypass if over 900 psi/ check with manufacturer on the specs of the machine. Regulator could also be bad/worn. might need to be rebuilt/serviced 

Call mike chem Tex phone tech he will get you through anything (832) 216-7027 

This machine uses a capillary tube and parker skinner valve to launch heat to the waste tank. The system has an 01 jet continuously spraying in the water box, 1200 psi with the waste tank lid on and pulling full working lift with the pump working that hard is exceeding the hp your motor realistically can sustain.

If you are doing tile and "think" you need 1200 psi switch to pressure washer hose for 1 job and run at 800 psi and reevaluate things. Flow runs spinners #facts 

To clarify Jim is saying use a 3/8 hose 

One blue summer... have you ever run high flow equipment? Carpet or tile.... asking for a friend?