That's a mechanical gauge, so I expect the tube is either crimped by engine movement or cracked, so that when you accelerate it leaks.
Check the tube from the firewall to the fitting on the engine (rear of cylinder head, I think). The braided cover tube to the right head is the coolant temp line.
There is no way acceleration will lower engine oil pressure....
I'd think the engine and transmission mounts probably need to be replaced as well.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!