Touch screen unresponsive sometimes

The only way to narrow things down is to try known-good replacement parts, one at a time until the problem is identified. You likely have a defective screen and the problem may actually stem from the original situation which caused Tristar to fail, assuming is was from some physical drop or damage. I would also inspect the connectors for any signs of physical damage.

The other possibility is that something was slightly damaged during the repair. It's not uncommon that too much heat gets transferred to the logic board while doing IC replacements and sometimes a component gets partially lifted from the pads. However, before checking that, I would try to eliminate the screen and connectors.