With the rev counter there's a brown lead going to the alternator. It was happening to mine , Maybe it needs to be pussed back on or wire stripped back & new connector put on.
Check all your pulleys are tight before fitting a new belt , Mine has shredded a few belts over the years and it's usually because a pulley is loose or worn.
There is a black relay under the steering wheel ,Maybe it's knackered.If you don't mind not having indicators just pull it out. I've changed mine a few times over the years.
Here you go ,Very quick delivery.:)
https://www.ebay.co.uk/ulk/itm/271839899718
Land Rover Discovery 1 LED Green ALL Dash Dial Instrument Bulbs 200tdi/300tdi/V8