I have a sundance tango (1996) with a single 115v 2 speed pump, that continues to shut off in high speed. The spa runs continuously on low speed (which it never did before) but will run for only several seconds on high before tripping off. During these few seconds on high speed the motor seems to be straining, and the flow does not significantly increase. Fifteen seconds later it will restart in low speed.
Some history: I recently replaced the circuit board after damage from a lighting storm. Everything worked fine for about 3-4 days. Then the problem started. I replaced the pump, that didn't help. The top side control temp sensor light went out, so I replaced that along with the transformer. That fixed the temp light, but still having the same problem with the high speed pump and the continuous running on low speed. Thinking that it may be a flow problem I replaced the filter... did not help. Any ideas on what may be causing this problem?

Sounds as though the low speed relay is stuck. When you call for high speed pump, the pack sends current to both low and high coils of the pump motor windings. The pump will sound like heck, then thermally trip off. If the relays do not have a dust cover on them, you should be able to see the low relay dis-engage when you call for high speed pump, or when you power down the system.
You will need to address the board.

inhotwater,
I believe you are right, the relay appears to be bad. I was finally able to get some time to look at the board. After shutting of power to the spa, I pulled the wire from the low speed relay. When I powered back up, the low speed pump was off (of course) and the high speed pump worked correctly.
Is there anything I can do to fix this other than replace the board (or put in a new spa pack)?
Thank you for your help,