1999 Lincoln Town Car Engine Stalling

1999 Lincoln Town Car 4 cyl Automatic 140000 miles

During acceleration the engine stalls in between 35 and 40 mph. In order to regain power I have to pull over and come to a complete stop and then I will be able to accelerate again. (The car never has to be turned off to be able to accelerate again) It has been doing this frequently and sometimes it will accelerate fine. Getting up a hill is also a problem. The engine turns over just fine no out of place noises. Before this started happening the A/C compressor was replaced. The CEL was on and a diagnostic test was ran and found problems with the catalytic converter and the spark plugs. Both catalytic converters were replaced and 4 of the 8 spark plugs were replaced. The remaining 4 spark plugs had been replaced in the last 2 years. A second diagnostic test was done and the error code T0603 showed up. A third diagnostic test was done but no error codes showed up. The third test also did not finish for some reason unknown. Any ideas?