1998 ford Escort zx2 Heat is slow getting warm. Temp gauge barely registers. I have reaplced the Thermostat, radiator cap, blew out heater core. No heat when in idle but works once driven. What now?

I owned 2000 zx2(used), first thing I ended up fixing/replacing was radiator, thermostat, and water pump, as someone decided to use stop leak to fix a leak in the system, sounds almost exactly what happened to my car. slow to warm up because restricted flow from radiator being clogged.if the cooling system ever got low enough it is possible that stop leak has entered the heater core and clogged it also. thing to look for is a yellow/green slime in system. this slime when it hits air hardens like a rock and there is almost no way to get rid of it except an acid flush(clr will remove also, do not leave in system) if stop leak has been entered into system.
best answer i can give with out seeing what it is doing,clogged radiator, bad water pump,or wrong/defective thermostat,heater core plugged

First thought, Thermostat is what determines coolant temperature. If engine is slow to warm up, you may have purchased a defective thermostat. Air coming out of heater vent when engine is fully warm should be around 120-130 degrees faren. Place thermostat in a pot of water on the stove. It should open when you heat the water to around 180.

Water pump. First make sure you have enough coolant and the reservoir system isn't leaking(or any leaks). Fords are hard to get air out of the system. I always over fill Fords because of air. It will usually purge out in a couple days. only getting warm at idle and hot going down the road is a water pump or something is still clogged up. IE.. heater core.

We have plastic impellers on the water pump that can and do separate from the impeller drive shaft. This is your most likely issue and you should stop driving this car until it is repaired since you would have reduced coolant flow through the engine. The engine could be overheating and you wouldn't know about it until it was too late.

It's a Ford. They are slow to warm up. All of my Ford products have been that way. Nothing was clogged. That's just the way they are designed.

The best thing to do is have your cooling system pressure tested, but based on description, sounds like a water pump, or intake gaskets