I have the answer, and my Spike is working better than ever.
It was the battery pack. Have someone replace the rechargeable batteries with the same or slightly stronger rechargeable batteries.
A lady at Fischer-Price suggested I find a hobbyist who likes to solve problems. I took my Spike to the local train store, where a hobby problem-solver examined him in great detail. Everything was working, including the little motherboard inside.
He replaced the battery pack. Spike is alive again.