With cars, some descriptions like the one you have here require a technician to attach testing equipment (pressure gages, and while not on this one it's too old but newer cars a scan tool to watch sensor inputs and computer commands) and then road test the car and experience the symptom first hand to figure out exactly what is going on. The biggest problem with this being a Volvo, is the lack of familiarity that most shops and techs will have with it. I average two Volvo visits a year, (same car, twice a year) so that's not enough exposure to even begin to have a chance at a similar transmission issue. I can only speculate in a very general way as to what "might" be occuring. One possibility is the fluid is foaming, which causes a loss of pressure. Some fresh fluid may alleviate the symptom for a while because of the additives in the fluid that help resist aeration. But the underlying cause, such as a torn seal (internal) will still be there and ultimately have to be dealt with.

If your husband suspects the filter is clogged and causing this problem one important note must be stressed. The filter only clogs when sufficient clutch and band material has failed. That means the tranny needs rebuilt, and a new filter and fluid will be wasted.

I am not familar with the Volvo transmission, therefore can only supply suggestions:

1. The transmission clutches are worn out. A rebuild or transmission swap is needed. After all, it is an '86 and presumably has more than 100,000 miles on it.

2. I'm not sure how much electronic control was present on your transmission, but may have had a temperature sensor. Some transmissions will be put into the "limp or run home" mode if the engine controller senses that the transmission is getting too hot, for example, climning a long grade. Therefore, if the temperature sensor fails or sends an erroneous signal, the tranny will seem to misbehave. However, in 1986, there may not have been much electronic control in Volvo's transmissions, and the shift valves may have been largely servo-hydraulic control.