I just put a rebuilt transmission ran good for a week . Now it runs at 10mph before I start to take it apart . What should I look into first.

. Ensure the PCM is good.

. Clean main PCM ground.

. Check electrical connections.

. Test all under hood and under dash fuses with a test light or multimeter:

https://www.askmehelpdesk.com/cars-t...tml#post252145

. Check fluid level.

. Drop pan and verify the filter/pickup is fully seated in the valve body of the transmission. The O-ring on the filter needs to seal properly. If the filter/pickup falls out, hydraulic pressures will plummet.