I had a very similar problem with an Mep-802a a couple of weeks ago. The 803 & 802 have the same engine... one just has two more cylinders than the other.
My problem was much more severe... started initially, ran for 12 seconds and quit. Took around 5 minutes to get it to start again. One it was running on one cylinder I got a lot of smoke 'till the other one fired up. Once fully running and no black smoke, it would generate 105% of rated load.
But if it sat for a day the restart problem came back.
The problem was the oil pressure relief valve was stuck.... or more properly, it was mostly stuck closed. After a while of running on one cylinder it would finally "unstick" and it'd run just fine.
There is another thread on this site that addresses the valve. Basically it is a simple and minor repair. Just tedious.
First you need a new "door" gasket... the door is on the oil filler/dipstick panel. Gaskets are available on the large auction site or just google the part number and you can pick it up elsewhere.
You will need 10mm and 13mm wrenches, a 24mm wrench, a block of wood (2x4) about ten or so inches long, a dead blow hammer a torque wrench that'll measure in inch pounds and a chopstick.
The chopstick is the best part; cause that means you are compelled to get a great chinese dinner--- ask 'em for chopsticks and take 'em home. I recommend Mongolian Beef with steamed rice and a vodka martini straight up.
Remove the battery on the left (fuel solenoid side). I usually remove the cable on the + side of the other battery, but that's just me.
drain the engine oil--- the more oil you can get out the better. Remove oil filter.
remove the solenoid and solenoid mounting bracket. Remove the two studs that hold the bracket to the block
remove the oil pressure line that goes to the sender --
remove the nuts that are across the top of the door... there'll be four of them. Be sure to remove these nuts before the next step
Remove the bolts that hold the door to the block.
I took the 2x4 and after removing the oil filler cap and dipstick I put the 2x4 on the oil filler opening and gave it a good whack with the dead blow hammer.
That split the door from the gasket and I was able to wobble the door off.
Now, there will be four studs (that had the nuts on them) still sticking out of the block....... DON'T TAKE THEM OUT OR LOOSEN THEM
They are part of the fuel delivery system and should not be removed. this is more than just a little important.
you can see the valve coming down from the oil pump scavenge. It is about 5 inches long and has a squared off housing that goes part way down the valve.
The 24 mm wrench will grab that sucker and you can unscrew it from the scavenge. It's tight in there so getting it out is a slow process.
On the bottom of the valve is a "C" ring clip. remove the clip. Be careful as that end internal piece is under spring pressure. It may pop out or may not depending on how gunked up the valve is. Mine was so gunked up that removing the clip didn't cause anything to happen.
The whole valve is in five parts, clip, housing, spring, piston, and the piece that goes on the bottom held in place by the c clip
once the C clip is out you can put the chopstick in the valve on the threaded end and you should be able to get it out by tapping the chopstick with a hammer.
Clean it up and put some assembly lube grease on the innards and re-assemble valve. Make sure that, once assembled, you can push the piston down with the chopstick and it moves freely. BTW, you can buy new valves - google the part number.
cleaning off the old gasket is not fun. Be sure not to remove those four studs -- it is tempting to do so in order to get to the gasket, but deal with it and leave those studs alone.
Reassembly goes pretty much the reverse of the disassembly.
Regarding those four studs... before assembly make sure that they are finger tight snug in their seats before replacing the door. They might tend to wiggle out a little during this whole process. There is a sequence for tightening those screws and four nuts, but I would put the gasket/ door on and at least finger tighten those nuts on the studs first and then finger tighten the bolts and the two studs that hold the solenoid bracket. on. then I'd follow the manual on tightening sequence.
The manual has the torque rating and sequence in it. think of it as when torquing head bolts except you are dealing with inch pounds instead of foot pounds
If it sounds complicated, it isn't. As long as you have the right gasket and tools and a full belly of Mongolian beef and rice it is quite easy
My thanks to whomever wrote the initial description of this on Steel Soldiers.