Air Conditioning - Drip pan overflow! Help!

We have a Lennox attic mounted A-coil and blower system. Last week, we had an HVAC contractor come in to clean the unit. They found that the coil was dirty -- nearly plugged, but, cleaned the rest of the system and scheduled the evaporator removal and cleaning. Round 1: Within 2 hours of their exit, we had water dripping from our kitchen ceiling. The tech had laid a heavy box on top of our PVC drain line, causing a deep sag in it. Water could not escape. That was an easy fix. Round 2: Yesterday, the tech's showed up, pulled the coil and blower, cleaned 'em, reinstalled it, and left. Within 2 hours of their departure, (you guessed it...), we're back to a rain forest in the kitchen. Called 'em, same techs show up and spent 3 hours, finally announcing, after much work with a Shop Vac, that they had cured the problem. Round 3: Within -- yep -- 2 hours of their departure, what used to be a drip in the ceiling has become a full fledged stream. At 2130 last night, the contractor's hot line got a very angry telephone call. And, they are due here within the hour...claiming that they are just 'gonna re-pipe the drain line.'

Question: Based on my observations, and, experience with this thing, is it possible that there is enough back pressure (suction) being placed on the 3/4" PVC drain line to prevent water from flowing thru the thing? I'm at a loss here -- and, I get the feeling that the HVAC folks are just waving their arms in the air, hoping for divine intervention, and, that the 'shotgun' approach to repair will be effective.

I'm ready to call my lawyer...I've been out >$900 and, for my efforts, have a ruined ceiling. Oh...and, my house has no air conditioning in August's heat!

Advice?