RUNS ROUGH STALLS CHECK ENG LIGHT ON

code p 0300 is random miosfires
indicates problem injectors from dirty fuel
Try a can of quality injector cleaner first in a tank to see it it smooths out
other causes include
ignition coil/s
HT leads
Plugs
Wiring but as it runs good at rpm and you have covered the rest , it is most probably sticking injector tips so use the cleaner or have the injectors serviced

Depends on what you mean by rough. Could be a cylinder misfire.They sometimes are not noticed as engine speed increases during driving or under light load as when idling in neutral but at idle under load as when stopped at a light in drive became noticable.
Could also be running rich. The causes of either are too numurous to list.Consult a good mechanic who can actually see and hear the problem to put you on the right track.

1996 isuzu bighorn, diesel the engine check light comes on and when it does the rev counter stops working and engine runs rough and stalls

you need to have the codes checked to know what direction to start repairs,Autozone does this for free

the carberator base gasket on these are notoriuos leakers. most auto parts stores carry just that gasket for about 5$.dont over tighten when reinstalling

You should start with the glow plug relay/timer.Check it to see if may be shorted.Remove the smaller wires going to it and turn the key back on,if the fuse does'nt blow then replace it,if it does then you need to unplug the fuel preheater in the filter housing and try the key on thing again,pretty sure it's one or the other.

Most likely air flow metre. Had same problem analysed and replaced and runs perfectly. Not cheap!

1. Vacuum leaks
2. Leaking egr or plugged pcv.
3 Air filter clogged
4.leaking head gasket
5. Timing chain worn, or jumped a cog.
6. Camshaft worn.
7. Valves burnt or leaking.
8. ignition system
9. injectors clogged
10. Idle control system

You'll just have to keep eliminating them, from cheapest to most expensive.

If it has a distributor cap, check for moisture inside when it is raining.

Change fuel filterS...plural
Don't forget the big canister unit under (next to wheel well)