I just did the same for a Marx 0-4-2. One of the 4 driving wheels has a traction tire.No reverse unit, but when it hits the isolated track to throw the switch (self tending), it noticeably slows. The jumper was wire and soldered. It didn't help.
It may be that the loco is losing power through the center sliding shoes losing contact momentarily as it traverses the switches. If you have a whistling tender, you can run a wire from the tender pickups to the locomotive. This will insure constant power to the locomotive as it travels through your switches.
The center pickups have continuity to each other so they are both connected, I would like to keep it stock but like Larry said, I think I need to connect the tender up. Have done this on other engines and don't like to do it especially since these things have been working for 70+ years. Well maybe they did.
Make sure both slide pickups are making good contact. I just had a problem where one, with no visible issues, was intermittent causing problems. A slight adjustment (e.g., bending) made all the difference. - now a smooth running loco. I sighted each pickup from the side of the loco (upside down) to see the relationship between the pickup height vis-a-vis the wheel face's contact with the rail.
The problem is the flanges on the wheels. The contacts and wheels are perfect on a straight piece but...on a 031 switch the wheels are lifted up where the plastic is. Modern locos don't have a huge flange like the 1689. I need to look at my 027 or vintage switches to see if those grooves are deeper. I think this along with a 70 year old e-unit make for a bad situation.
I have that problem with the newer Marx O-34 plastic switches. Decided to go with Gargraves for my wider radius loops. But loco's running on my Marx O-27 metal switches are smooth as silk. The early Marx design moves the rails. There's no frog at all.