I hate to say it, but it appears your pump preasures are inadiquate when the oil is hot. Unless you have a clogged return filter, that is. Hope it's the filters. Other wise it sounds like your in need of either pump or motor rebuilds, or both. First step is to check the pump out put preasures when the machine is up to temperature. IF the pumps are OK, then the wheel motors are bypassing too much oil and are in need of rebuilding.
When you say it looses power, does the motor pull down, or not? If not I am really sure about what I have said. If it pulls the motor back, it could be engine related, not the hydorstats. Again this is probably better than the pumps and motors, cost wise to repair....
Good luck, Kirk