AGR Valve: Check Function & Repair

Hello,

I have an Omega 2.5TD, manufactured in 1995 (396,000 km, automatic transmission at 195,000 km), and I'm still having problems with my EGR valve. I found a valve where the connector was slightly damaged (I had to file the connector on the valve to make it fit properly). When I plug this connector in, the Omega runs very poorly. It sputters when starting (and there's a thick black smoke cloud on the road). Once the engine reaches operating temperature, it has extremely poor performance in the lower RPM range, and the engine runs very unevenly. In the higher RPM range, it performs well, but with a lot of noise. When the valve is not connected, the engine runs more smoothly, starts well, and also runs relatively quietly, but unfortunately, it still has a significant loss of power in the higher RPM range. I received some helpful tips and, based on that, I disabled the EGR valve. Now, it doesn't matter whether the connector is plugged in or not; the Omega runs acceptably.
I would like to restore everything to its original condition. I'd like to either have the AGR valve repaired or, as a first step, have this vacuum valve checked. However, I don't know how to do it because I lack some technical knowledge. Could someone perhaps explain how I should proceed? Is it possible to clean the AGR valve?

Thank you in advance for any help you can provide.