1998 Dodge Neon P0134

Hi,
It all started with my engine overheating, I was long way from home so I drove it anyway stopping on the way or turnig the heater on to pull the temprature down. When I got back the codes were for both the o2 sensors, the mechanic replaced the thermostat, after about 100 miles the check engine light came back on with P0134, so I replaced the upstream O2 sensor, since then the check engine light started coming on with the same code after every 5-6 miles, but then it goes off after a few miles, Also the car was losing power in the morning after going a few miles, the mechanic suggested that it was the CAT, I got that replaced too, now the power is back, however the check engine light comes on every 5-6 miles & then goes off by itself, each time the code is P0134, please help.