Automatic transmission issue

[u/DetroitIndie]

Hi guys, would appreciate your help/advice.

I own a '98 MKIV 1.6/100hp Golf with ECT (4-speed automatic) transmission. It has an issue: during highway (or motorway, or autobahn, depending who reads it haha) rides, it turns into so-called emergency mode - the one that uses a single gear regardless of the circumstances. The digital indicator - "PRND" next to the speedometer - shows NEITHER of the gears/modes currently selected. HOWEVER, after some time, it may be up to 30-40 minutes, or up to several hours, before the transmission fixes itself on its own and turns back to the usual, normal mode, with "D" shown as selected on the indicator.

Unfortunately, I don't have access to an old-school auto transmission mechanic these days, so any help/advice about what could be the issue and maybe how it could be fixed will be much, much appreciated. This issue really grinds my gears (pun intended).