I have a 1999 Chev. Suburban 5.7 litre sfi. After the engine warms up, the ignition timing doesn't seem to be responding under a load. If you accelerate slowly, it runs fine. If you accelerate quickly, it spits and sputters and backfires. I have replaced the tps. The ignition module and pick-up coil tests fine. An Autozone scan indicated PO452 code (which was showing up a good while prior to symptoms). The symptoms first showed up during a long trip then not again for several weeks. It has now gotten to the point that it does it every time it warms up. Cooling down the engine and restarting it runs fine again until it heats up. Any ideas or suggestions?