Loss of RPM issue

Hello;

New to the bobcat/skidsteer, however, somewhat mechanically inclined, and have so far been stumped. So here goes:

Older bobcat 743 maybe from the 80's
has a 30 HP Kubota diesel motor (engine model V1505-T-E3B)
has had several modifications over the years so just keeping it running is a challenge. Here is the issue...

At idle, it runs fine, for as long as I am using it.
If I kick the RPM up a little (maybe 500 RPM from idle) and use it for more power it runs for about 10 minutes then the RPM drops down to the point where it barely idles. Bucket & boom both move barely, and hardly moves out of its own way.

Here is what I have done:
First thought it was overheating so I added an aftermarket radiator fan, and connected the regular fan (both 12v) directly to the battery. (it has an electrical issue where none of the gauges are working at all, will get to that later).

That was not the solution

It had an exhaust tube from the motor to the muffler (flexible exhaust) that was broken and was leaving a black stain on the inside of the back door, replaced that.. thinking that the intake was getting too much exhaust fumes back to the intake side of the motor...

That was not the solution

Changed the hydraulic filter and added about 3 gal of fluid to the tank

That was not the solution

There was a "funky" setup for the air intake filter (rubber radiator hoses cobbled together to a large 3" intake and 10" rectangle aftermarket air filter, the filter was black from the exhaust leak) removed that, ran with no filter (until I can get a proper replacement) still had same problem. (no intention of running it for long without an intake filter, very bad, I know...)

That was not the solution

I am suspecting that it might be some sort of a fuel/heat issue since I have repaired/replaced everything else, but if it was something like a filter, then my suspicion is that it would run with an RPM issue all the time. Is there some sort of pickup, or filter in the tank or fuel line issue that I can look at?

I know that this thing sounds like a POS, but I got it on a trade for an airplane, and I am only using it for personal use on my land to modify and build a shooting range. Other than the RPM issue which has been an on-going issue since the beginning, it is a pretty good little machine. Has done some really good work for me so far. I have no intention of using it for hire, I would just like to be able to run it for several hours with no issues so I can blow out the work that I need done over a couple of weekends. (I know, I am dreaming..)

Any suggestions?

Thanks for your help

Attached Images

 

Loss of RPM issue