91 MB 300TE gas losing power

Looked at a clean 91 Mercede 300TE (gas, wagon) tonight. Owner says it has been losing power progressively over he past few weeks. Ad states, in part: "I bought it and put a new water pump in, new OVP relay, new K&N air filter, transmission filter, plugs, cap/rotor button, fuel filter. Car won't go over 45 mph." I don' tthink it will exceed 35, actually. It starts (with difficulty) and runs. ABS light is on, and cold idle is not smooth, pointing toward the OVP relay, but that did not cure the problem. Idles fine after a minute or so and trans shifts normally. Running and in neutral it starts missing around 2700 rpm and will not rev past 3000. Feels like a fuel delivery problem to me. The timing chain is noisy but I don't think it's jumped time as it is hitting on all six. Owner has a receipt for full exhaust 2 yrs ago so I don't think its a bad cat. I'd like another wagon in my driveway and am looking for ideas about what might ail this one. Interior is near mint, paint is good and It could be had cheap; might be worth a fix if cost effective. Ideas?

I was going to guess clogged cat.

Vacuum gauge.

compression?

I had the exact problem with an Evo and it turned out to be a fuel pump on it's way out. Wouldn't rev over 3000rpm under boost and would barely go over that if you really light footed it. I pulled the fuel feed to the rail and jumped the fuel pump, it took way way too long to fill the bucket.

If only I would have checked the fuel pressure at the rail I could have saved a lot of time. I'm not familiar with the 300TE injection, is it easy to hook up a fuel pressure gauge?

It wishes it were a diesel?

Similar to the issue I had on my W126. Replace both fuel pumps, and the fuel pump relay just to be safe. It's POSSIBLE that it's the EZL but start with the fuel pumps.

Thanks for the suggestions.