Fuel gauge not working all the time.

This is a normal concern.
The instrument cluster uses 4 different operating modes to calculate the fuel level:
Anti-slosh (default mode)
Key OFF fueling
Key ON fueling
Recovery
After a fuel fill up, the time for the fuel gauge to move from empty (E) to full (F) ranges from 2 seconds to 55 minutes depending on which operating mode the fuel gauge is in.
The default fuel gauge mode is called the anti-slosh mode. To prevent fuel gauge changes from fuel slosh (gauge instability due to changes in fuel sender readings caused by fuel moving around in the tank), the fuel gauge takes approximately 55 minutes to go from empty (E) to full (F).
The key OFF fueling mode (2 seconds to read empty [E] to full [F]) requires 3 conditions be met:
The key must be in the OFF position throughout the entire refueling of the vehicle.
At least 15% of the vehicle's fuel capacity must be added to the fuel tank.
The instrument cluster must receive a valid key ON fuel sender reading within 1 second of the key being put into the RUN position. The key ON sample readings are considered valid if the fuel sender reading is between 15 ohms ± 2 ohms and 160 ohms ± 4 ohms.
If these conditions are not met, the fuel gauge stays in the anti-slosh mode, which results in a slow to read full (F) event.
The key ON fueling mode (approximately 90 seconds to read empty [E] to full [F]) requires 3 conditions be met:
The transmission is in PARK (P).
The key is in the RUN position.
At least 15% of the vehicle's fuel capacity must be added to the fuel tank.
In key ON fueling mode, a 30-second timer activates after the transmission is put into the PARK (P) position (automatic transmissions). When the 30-second time has elapsed and at least 15% of the vehicle's fuel capacity has been added, the fuel gauge response time is 90 seconds to read from empty (E) to full (F). When the transmission is shifted out of PARK (P), the fuel gauge strategy reverts to the anti-slosh mode. The key on refueling mode prevents slow to read full events from happening if the customer refuels the vehicle with the key in the RUN position.
Recovery mode is incorporated into the instrument cluster strategy to recover from a missing fuel level input during a refueling event. Missing fuel level inputs result from intermittent opens in the fuel sender or its circuits. Recovery mode (empty [E] to full [F] approximately 20 minutes) is initiated when the following 2 conditions are met:
The instrument cluster is in the anti-slosh (default) mode.
The actual fuel level in the tank is greater than what is being displayed by the fuel gauge

Article No.
95-2-3 01/30/95
^ GAUGE - ERRATIC DISTANCE-TO-EMPTY AND FUEL GAUGE READINGS - VEHICLES BUILT BEFORE 1/1/94 ^ GAUGE - FUEL - ELECTRONIC - "CO" DISPLAYED IN ELECTRONIC CLUSTER WITH TWO TOP/BOTTOM BARS HIGHLIGHTED - VEHICLES BUILT BEFORE 1/1/94 FORD:
1986-92 TAURUS
1989-94 PROBE
1990-94 CROWN VICTORIA
1993-94 THUNDERBIRD LINCOLN-MERCURY:
1985-92 MARK VII
1986-94 CONTINENTAL, SABLE
1990-94 GRAND MARQUIS, TOWN CAR
1993-94 COUGAR, MARK VIII LIGHT TRUCK:
1990-94 AEROSTAR ISSUE:
There may be erratic Distance-To-Empty (DTE) and fuel gauge readings, or circuit open "CO" displayed in the electronic cluster with two (2) top/bottom bars highlighted. This is caused by fuel sender card overprint material and/or an intermittent connection between the spring and contact on the sender card.
ACTION:
Install a revised sender assembly or fuel pump/sender assembly. Refer to the appropriate vehicle Service Manual, Fuel System Section, for removal and installation procedures. Refer to the Parts Block in this article for correct parts usage. OTHER APPLICABLE ARTICLES: NONE
WARRANTY STATUS:
Eligible Under The Provisions Of Bumper To Bumper Warranty Coverage For 1992-94 Models And 1991 Lincolns, Major Component Warranty Coverage For 1990 And Earlier Model Lincolns, Basic Warranty Coverage For All Other Models OPERATION DESCRIPTION TIME 950203A Replace Fuel Gauge 1.5 Hrs. Sending Unit - Town Car 950203B Replace Fuel Gauge 0.9 Hr. Sending Unit - Crown Victoria/Grand Marquis 950203C Replace Fuel Gauge 1.0 Hr. Sending Unit - Taurus/Sable 950203D Replace Fuel Gauge 1.2 Hrs. Sending Unit - Mark VIII 950203E Replace Fuel Gauge 1.1 Hrs. Sending Unit - Probe 950203F Replace Fuel Gauge 1.2 Hrs. Sending Unit - Thunderbird 950203G Replace Fuel Gauge 0.9 Hr. Sending Unit Mark VII 950203H Replace Fuel Gauge 1.2 Hrs. Sending Unit Aerostar DEALER CODING BASIC PART NO. CONDITION CODE 9275/9H307 G25 OASIS CODES: 204000, 204100, 206000