Possibly just air in the coolant, perhaps give it a good burping!

 

The coolant temp should always be above 180F after a few minutes, otherwise a bad t-stat and maybe an obstruction in the cooling system.

 

Burping done, ETC sensor replaced. I have not replaced the t-stat yet, didn't have one locally. I didn't take the car more than 20 miles all weekend but, I went back to work this morning and was 40+ miles away from home and I lost 4th again. Ambient temp was 56F, car was running about 176-182F speed was set at 71mph. I got to work and disconnected the negative cable all day (11 hours). Reattached it and drove home with no issues. I did find another thread on a FB HHR group that matches quite a bit of what I have going on. I did check the codes again on my way home today and I found the fallowing codes:
P1818 Transmission fluid pressure (TFP) valve position switch indicates drive without drive ratio,
P0562 System low voltage,
PO563 Fluid Temperature (TFT) sensor performance,
PO716 input speed sensor performance,
PO717 input speed sensor circuit low voltage,
PO722 Output speed sensor circuit low voltage,
PO723 Output sensor circuit intermittent,
PO741 Torque Converter Clutch (TCC) system stuck off,
PO742 Torque Converter Clutch (TCC) system stuck on,
PO751 1-2 shift solenoid valve performance (no first/fourth gear),
PO752 1-2 shift solenoid valve performance (no first/third gear),
PO756 2-3 shift solenoid valve performance (no first/second gear),
PO757 2-3 shift solenoid valve performance (no third/fourth gear),
PO842 Transmission fluid pressure (TFP) switch circuit low voltage,
PO843 Transmission fluid pressure (TFP) switch circuit high voltage,
PO961 Line pressure control solenoid system performance,
PO973 1-2 shift solenoid control circuit low voltage,
PO974 1-2 shift solenoid control circuit high voltage,
PO976 2-3 shift solenoid control circuit low voltage,
PO977 2-3 shift solenoid control circuit high voltage,
P1810 Transmission Fluid Pressure (TFP) position switch circuit,
P2534 ignition 1 switch circuit low voltage,
P2763 Torque Converter Clutch (TCC pressure control solenoid control circuit high voltage,
P2764 Torque Converter Clutch (TCC pressure control solenoid control circuit low voltage,
U0100 Lost communications with ECM,
U0121 Lost communications with Electronic Brake Control Module (EBCM),
U0140 Lost communications with BCM

There seems to be a lot going on here. TCM issues maybe? Thoughts?

 

You could try cranking down on those 4 bolts inside the underhood fuse block.

 

Were those codes current? CEL? If not, result from battery disconnect.

 

Not sure, I cleared everything and car does fine for a day or two then starts acting up again.

 

The four bolts in the under hood fuse box? Did you crank them tight?

 

Update on this slipping issue. I know it's been a while but, I wasn't able to work on it for till recently.. I found a work around that lasted quite a while to keep the ol' gal on the road.. I just had to disconnect my negative battery cable. The issue would correct for a while and as long as it was disconnected for 20 minutes or more I didn't have any problems until about a month ago. I would disconnect my battery every day when i got to work and again at home over night. About a month ago, this stopped working and I ended up not being able to reset it and having to drive 30-40 miles a couple times a day back and forth to and from work. I finally got in the garage a couple weeks ago and dropped the pan and pulled the filter and side cover. replaced the TCC, put in a new filter and fluid and it's back on the road running and driving great. No shifting issues at all. I apricate all the advice!

 

Was prolly low on fluid.