OK, try to visualize this:
the motor is gummed up inside just enough that the power applied is not enough
to overcome BOTH the inertia and the tiny bit that they gummy lubricant is
causing it to stick. If you whack it a couple of times, you will break the
rotors free of the gumminess, allowing the motor to start. Once the motr is
started, it will operate normaly, UNTIL it is left off long enough to get
stuck again.

We saw this with hard drive a LOT back about 10-15 years ago.
Drives would run fine for years, then get turned off and not ge able to start
again. We would carefully whack them in the right place and get them started
again, then copy off all the data, since it was just going to get stuck again.

The problem is the Brushes in the motor are more than likey Worn out
and not contacting the Armature. OR there is too much carbon dust
buildup on the armature. This is a common issue I've seen in Ford
convertible motors. Even Crystler I think uses the same type of motor.

The easiest way to fix this is from the trunk look were the Motor/Pump
assembly is near the back of the rear seat. The motor portion is to the
drivers side. You should see 2 bolt heads on that side. You should also
see a ground wire going from the motor portion to the chassis along
with a Red and Yellow wire that feed into the Motor.

First thing you want to do before removing the motor however is make
sure you are getting power to the motor. Using a multi-meter is the
best method. To test you should see very near-by(again to the drivers
side) the 2 Relays that operate the motor. Feeding to each of those
relays is a Black wire with a Yellow stripe. Those should always be hot
either with key on or off. They are a direct feed from the power
distribution block. Once you locate those wires remove the relays from
the connectors they are attached to. They're the same so it doesn't
matter which goes back on which connector. Again using your notes find
whild slot in the connector is fed to the Black w/Yellow wire. Place
the tip of your positive test lead from multimeter(set to DC Voltage)
in that slot and touch the Neg test lead to chassis ground. You should
get a 12vDC reading. Test both. If both show voltage your good to go
with power. Put the relays back on the connectors so you dont lose
them. It may be a good idea to test the Ground lead for continuity
between the motor and the chassis to make sure there's a good ground
contact. It's the Black wire running from one of the bolts on the Motor
to the chassis.

Now back to the Motor. To remove from the pump.
1) Unplug the Yellow and Red wire connector from the Motor.
2) Remove the 2 Bolts holding the Motor to the Pump.
3) Carefully slide the motor out and clear. Caution here. There are
some very thin metal washers used as spacers on the input shaft of the
motor. Don't loose these.

Once you have the motor out you will probably see ALOT of dust and gunk
caked on to the motor windings and anywere else it could get to. Get
yourself a spray can of Electric Parts cleaner, available at just about
every Auto parts store. You want to spray the heck out of this thing.
Get all the balck dust out of the motor, spray until it runs clear.
Now near the top of the motor you'll see a plastic piece that contains
the Brushes with springs behind them. If the brushes look really small
and aren't always touching the armature thats your problem. You will
need to aquire new brushes. To install new ones you will need a
Soldering Iron.

I've replaced these brushes in these motors a few time using the same
brushes that AC/Delco uses in there 75AMP Alternators thats installed
in just about every 87-93 Fox body Mustang with a 5.0 Liter engine. So
when your hunting down brushes for these motors there is a good place
to start looking at your local auto parts stores. Take the brush holder
piece(it pops right off the motor once you un-solder the ground lead)
with you to the Store and trial fit before buying the brushes. Most
replacement brush sets run from $4 to $5 dollors. Once you have a set
of brushes and get them reinstalled you'll be glad you did this. A
new/refurbished pump motor can run over $100. Talk about saving your
self some money.

Oh I should also mention it's a good idea to take some emery paper and
lightly sand off the Armature so it has a nice bright copper look to
it. Also clean out the area were the motor slides into pump. Get it all
nice and clean so you don't prematurely wearout new brushes.

Good luck to you and I hope all this helps.