A6 intermittently loses power and heat after driving a while

I have a 2000 a6 2.7t. I live in a small town and daily driving to and from work about 5 miles each way my car doesn't get up to normal operating temp even if I run it for 15 minutes before I leave the house. Replaced the thermostat last year but it is still having this issue. The car only heats up to normal temp when I go on a long trip out of town. The last two times I have taken it out of town it has started to intermittently lose power. I will be driving at a reasonable rpm and suddenly it will feel like it lost power and lurched like I just downshifted. The last time this happened last week I decided to shift into neutral and rev the engine in case there was some buildup on a spark plug. It seemed to help, but then a few minutes later the car lost power again. Then I noticed the heater was blowing cold air and both the oil and water temp gauges were way low. I turned off the heater, and the gauges raised up to normal levels. I could then get heat again, but the gauges would go back down and then the air would cool off again. During all this, every few miles the car would feel like it lost power or downshifted. Took it to my local mechanic the next day and throwing no codes. Help please!