1987 ford mustang 5.0HO

if it runs ok warm and not cold then i'd go for a small vacuum leak. When your car is warm the computer is richening up the fuel mix to compensate for it. Or it can be an ignition problem.

Now it pops only when the engine is under a load, when i rev it up in park or neutral it does not pop, the poping seems to come from under the plenum. I have checked vacuum lines and most are pluged up. Main supply, brake power booster and ac is going to the t, one of the vacuum line from under the plenum thin metal ones of the two is going to the fuel pressure regulator. The timing i had to advance to 13 Degrees because it's running like it has a vacuum leak or not running on all cylinders.
PLESE HELP

I'd bet you aren't telling the whole story here. I'd bet this has had a high performance cam in it and the engine is actually running to lean andneeds tobe richened up to stop the popping, unless yu have not got the valves adjusted correctly. You reason for adjusting the timing so high seems to be due to a different cam as a stock cam would not require that. If it is a stock cam and you have advanced the timing then check for a worn timing gear/dist gear, or iginition prpoblem like a plate in the dist worn or if you have vacuum advance, either rust at the pin oradvance rod worn out.