Before checking injection, check ignition. Before ignition, check mechanical problems.

1st, Set tappets to .022", then do a compression test.

Check ignition timing at idle with no vacuum hoses connected to dizzy, it should be 10°, rev it to 3500rpm and it should be 34°. Put timing light on number 6 and check if it does the same.

Are the plugleads original? Cheap silicon replacements are only good for 2 years, then performance drops markedly.

Pull the plug leads boots back to bare the connection to the spark plug and short them out one at a time with a screwdriver 1st contact the tappet cover, then slide it down to contact plug.

Fit 6 new NGK BP7HS.

With a big screwdriver, check if the idle mixture screw on the ECU does anything to the idle.

Does the car go OK on the road when you run it at cruise speed?

Still driving, slowly squeeze the pedal to the floor, does that cause any problems?

Does the car go OK on the road when you hold it flat?

You should disconnect the vacuum hose to the MPS and suck on it, making sure the hose doesn't have a hidden crack. A leaking MPS should not affect the idle, but affects part throttle performance seriously and will eat a mountain of gas.

The thermostat will have little to do with your problems, EXCEPT, it indicates yours is not closing the bypass properly. The head distributes the cooled water via the brass pipe in it.