'91 Ford Escort 1.9L Manual Trans runs rough ONLY under heavy load

Around 1996 or 1997 the Check Engine Light will come on and store codes that are very useful in diagnosing drivability problems but prior to this the computer was a lot less sensitive. Because the timing belt broke I would ensure the engine is mechanically sound by first checking engine vacuum with a vacuum gauge at idle should be 21 in HG with a warm engine. I would recommend doing a cylinder leakage test where compressed air is put in to each cylinder one at a time when the Intake and Exhaust valves are closed 100 psi of air is put and the amount of air that leaks out is measure to see if the valves are not sealing properly (possibly the valves may be slightly bent when the timing belt broke). A compression check an be done to see if the compression is even but a cylinder leakage test is more valid. If engine vacuum is down at all it will drastically effect the engines driving characteristics. Would love to hear what you fine to be the problem. The repair shop are right in a sense in that they had no way to run the engine first when the car came to them.

Have 1994 Escort with 275,000+. Try checking the spark plug wires on a dark, moonless night. Open the hood & rev the engine to about 3000 rpm. If you see sparks from the boot to the sharp edge of the head or a pretty blue glow (ionizing air); replace the plug wires. Chief auto parts has EXCELLENT plugs with lifetime warranty. I've replaced them twice. Ford's aren't warranted and fail within a few 10K, same with others. No I don't own Chief auto parts store or stock! ;-) (Actually buy most stuff @ Autozone)

you may need to take it to a mechanic and have them check the timing of the engine this could cause it to mess up especialitly after you just got a new timing belt