I have a 93 Plymouth Sundance that has been running good until just a few days ago. When I now start it up, when stepping on the gas to head down the road, I can hold the pedal to the floor and it will barely get up to 15 mph, struggling to get there. Once it does and I attempt to increase my speed, at about 35 MPH it starts to cut out and almost dies. Someone said my thermostat could be stuck open, or my catalytic converter plugged. Any suggestions please? It was a great work car until this started.

disconnect power supply to fuel pump, then start engine when engine stalls fuel pressure released

any time you replace the fuel pump you MUST replace the fuel filter. Sounds like it could be a couple of different things. Pick up coil if it has a distributor. bad coil pack. Replace fuel filter first .Good Luck!!!!

it is more than just the valve seals.the 3.0l engine is notorius for the valve guides slipping down into the valve pocket. there is a fix that involves adding a keeper to prevent them from falling but it requires removing heades and taking them apart. yours may have already have had this done so valve seals may fix it.
yes you can change without removing heads you need to have piston at the very top off stroke if off by a little the piston will be forced down by the air pressure so somehow locking crankshaft from moving would be a good idea.
remove plug from a cyl and fashion or buy a fitting to screw into plug hole that will also hook up to a air hose from a air compressor. the air pressure holds the valve closed then you will meed a valve spring compressor designed for removing the springs with head still installed. can rent them at auto parts stores.remove spring and replace seal. then reassemble and move on to the next
but sometimes the valve opens or piston moves down and the valve falls into the cylinder. you can pull it back up and try again or just take the heads off.
my recomendation is just take the heads off they are fairly easy to remove and instal. then have machine shop cut the grooves in the guides and install keeper clips

sounds like a fuel problem, fuel filter clogged, or low pressure on fuel pump.

Hi I am vortash to check if the cylinders are getting fuel just turn the vehicles engine few times then remove a spark plug to see it it is wet if it is not a fuel problem more likely spark failure replace plugs as they can fail under pressure and check for spark ,, may als be a head gasket problem so check compression and see if above 140 0r in the green with each cylinder . hope this helps regards vortash

mass air flow censor

Check all your vacum lines and make sure that your plug wires are properly secured this seems like a minor problem

Turn key, on, off, on, off, on & leave it on (not start) & count check engine flashes. 2 digit codes, for example: flash, flash, flash, pause, flash,flash., is code 32

check you r gas lines from tank to motor,fix leak 1st ,if it still idles high let us know