SLOW TO START , DRAGGING

Cooling fan(s) probably aren't working. Highway speeds provide the airflow necessary to cool the engine. Slower speeds do not.

I suggest running a scan on it.
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If it floods, then open the throttle body all the way. this gives more air and should start. If it is slow to start, the starter turning slower, then replace the starter. It is dragging. It is a common problem on older starters.

You have a sticky "thermostat" valve.
It should stay "closed" until the engine warms up,
then open, to allow more coolant to cool the engine down.
If it is "slow" to open, your engine gets too hot, until it does open.
Then, the coolant cools down the engine, until the thermostat finally closes again.

when the tank gets low on fuel(under 1/4) or less.. the pump, which uses the gas to keep cool, will get hot and start to cavitate(run at slower speeds and cause a severe fuel pressure surge)..the engine will respond accordingly..most will do this as the system gets older..to avoid it and possible premature pump failure, keep the tank at least 1/4 fll..

I would say your heads have overheated and are likely warped. I would shop around for another engine.

I have seen this problem before and the cause could be TCC Staying in Lock-up or an Electrical Issue within the Engine itself or a Fuel Delivery Issue.

If the Transmission TCC is the issue, try driving at slower speeds (below 30 and come to a stop, does it Shut off then? Increase Speed by 10 and try again. Does it react differently once you go above 42-45 MPH?

most likey if you changed the thermostate and gasket and antifrezze and water your fan clutch has gone bad an needs to be replaced the clutch works on a friction device the hotter it gets the more the friction plates ingauges causeing the clutch to turn much faster thush cooling the engine. Without this cooling action this will destroy the engine and trans misssionsdm without this cooling so this is true to this date the cooling in fan fan has to work to be able to keep engine cool. now days they call it a puke tank to keep from burning up the engines and is designed so that hot water is pushed out of the tank into the overflow tank so that when it gets hot there us bo waiting for the fluid to be sucked intothe radiator it's done automatically and saves the engines. So small changes have stuck with us and saved the lives of people in the miliatry and in the non milataray systems..

I would definitely consider the fuel pump. I had long turnover times before starting for years, but even worse was in hot weather it would just stall while driving. Sometimes it would only idle, but not run if you pressed the gas very far. Park it for 20-30 minutes and it would run again without another problem for weeks. Dealers and mechanics kept replacing other things like ignition coils and distributors. After a conversation where someone said they had the same problem, but only when they had less than half a tank of fuel, I knew it was the pump. I replaced it myself and all problems went away.
The fuel leaks back down to the tank and air gets in the lines. Then the pump is slow to build pressure up again. If you turn the key on, but not start, wait 45 seconds, off, on again but not start, wait again, now start it will take a little less time to start, less battery drain, until you fix the problem.
You may not have the heat problem in Canada. But here in Georgia it gets hot enough that if the fuel tank is low and the pump has this problem, it will overheat and shut down till it cools. A full tank helps it keep the pressure up and serves to cool the pump.

It could be the starting motor is faulty or was not installed correctly, particularly if you are finding it turns over the engine slower than it should (dragging and dragging down the battery voltage). When starting the car, the teeth must mesh properly with the ring gear or all sorts of strange things will eventually happen. Alignment is controlled by shims on the mounting bolts and if they are the wrong ones or missing, also usually bad news.