Abrupt gear change of automatic transmission

Bangs as in makes a noise, or bangs as in going into gear abruptly - not a smooth change?

Check the transmission fluid level, but you probably need a transmission rebuild, maybe a failing bearing. go to a shop that specializes in transmissions. They can connect a pro grade scan tool and diagnose all the shifts.

It appears to be a known issue.

https://www.google.com/search?q=1999+Volkswagen+Bora+V5+automatic+3rd+gear

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Automatic Transmission Shift Lock Control System
The automatic transmission shift lock control system is a safety device that prevents an inadvertent shift out of PARK when the engine is running. The driver must press the brake pedal before moving the shift lever out of the PARK position. The system consists of the following components:
• The automatic transmission shift lock control solenoid
• The automatic transmission shift lock control switch
With the ignition in the ON position, voltage is supplied to automatic transmission shift lock control switch. Voltage flows through the normally closed contacts of the automatic transmission shift lock control switch to the automatic transmission shift lock control solenoid. The automatic transmission shift lock control solenoid is permanently grounded. This energizes the automatic transmission shift lock control solenoid causing the transmission shift lever to be physically locked in the PARK position. When the brake pedal is pressed the contacts in the automatic transmission shift lock control switch open, de-energizing the automatic transmission shift lock control solenoid. This allows the shift lever to be move out of the PARK position.

Circuit Description
The transmission range (TR) switch is part of the park/neutral position (PNP) and back-up lamp switch assembly, which is externally mounted on the transmission manual shaft. The TR switch contains four internal switches that indicate the transmission gear range selector lever position. The powertrain control module (PCM) supplies ignition voltage to each switch circuit. As the gear range selector lever is moved, the state of each switch may change, causing the circuit to open or close. An open circuit or switch indicates a high voltage signal. A closed circuit or switch indicates a low voltage signal. The PCM detects the selected gear range by deciphering the combination of the voltage signals. The PCM compares the actual voltage combination of the switch signals to a TR switch combination chart stored in memory.

These are all electronically controlled , yur best bet would be to take your vehicle to the dealer .

What make vehicle ? Most all vehicles have a safety system .Don't try an force anything you'll break it !
Electronic Park Lock (EPL)/Ignition Lock Cylinder Control Actuator: If the vehicle is equipped with automatic transmission and a floor mounted console gear shift, it has an ignition lock cylinder control actuator system in the steering column. The ignition lock cylinder control actuator purpose is to prevent the ignition key from being turned to the OFF position when the transmission is in any position other than PARK and the vehicle may still be moving. The column ignition lock system consists of an ignition lock cylinder control actuator, and a park position switch that is located in the automatic transmission shift lock control switch. The ignition lock cylinder control actuator contains a pin that is spring loaded to mechanically prevent the ignition key cylinder from being turned to the lock position when the vehicle transmission is not in the PARK position. If vehicle power is lost, and/or the transmission is not in the PARK position the operator will not be able to turn the ignition key to the lock position and will not be able to remove the ignition key from the column. They also have this on column mounted shifters !

Sounds like the transmission is shifting into both reverse and a forward gear at the same time. Time for a tranny overhaul. But first check the transmission fluid level.

The dash lights are powered by the battery through the ignition switch via a fuse. Check the electric connections and conditions of the wires.

Change the transmission fluid to solve transmission issues. Original fluid is always preferable as it is specifically designed for the transmission.

i would change the ****** filter see if that fixes the problem!

run the fault codes to find the reported fault
engine lights report on engines and transmissions
could be actuator or vacuum to actuator, dirty transmission oil or pump pressure problems

Hopefully this Transmission is still under warranty?
If its a stick the shifter or schronizers could be the culprit. If the vehicle is an automatic,shift solenoid, shifter, problems in 2nd and 3rd circuitry (band or clutch not releasing properly ),manual shift valve .
Take it back to whom ever did the rebuild or transmission work tell them the problem .
Business is so demanding these days they will probably do the right thing.

these cars are prone to motor/transmission mount failure. This can lead to erratic shifting. No guarantees but I have seen this a few times.

have you checked the brake lights for operation ?
if they don't work than it is your brake light switch located on the pedal support bracket.