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Sounds like you better get to a transmission shop and have those codes pulled. If the O/D light is flashing the transmission has a stored trouble code. The cheap parts store code readers won't do it so you'll have to have a good shop read it for you. Post up what they find and we can speak a lot more intelligently. Until then we're just guessing.
Check the wiring harness coming off the solenoid pack. My truck had the exact same symptoms recently, and it turned out to be that harness. You can get replacements for around $30 but you'll need to splice as they only include the plug on the solenoid side.
I cannot emphasize enough to check and clean ALL wiring connections from the transmission to the front of the engine...computer ( PCM) too .... Before removing the pan and solenoid pack....It has to be replaced as a unit..I went through all this bull**** about 2 months ago...My solenoid pack was toast ( EPC solenoid) but I also had alot of corrosion at the trans connector and at the front of the engine.....Thes guys have a decent price on the Solenoid Pack E4OD, 4R100 Transmission from PATC, E4OD, 4R100
I agree with checking the wiring first. The plug/harness that connects to the solenoid on the passenger side are know problems... possibly due to the heat from the exhaust.