HOLD on to your seats and prepare for another wild ride on the stock exchange this week was the advice from brokers yesterday. The Hang Seng index's short-term future was far from certain and the current volatility looked set to continue, they said. Friday's record 597-point climb may have cheered investors but the ''sell'' hawks are still circling the market. There is broad agreement among analysts that 10,000 should represent a firm support level, and many expect the upward momentum to be resumed in the coming weeks. However, none were prepared to predict a level for the index by the end of the week. ''It is day-to-day at the moment. It's very difficult to pick what will happen in the short term,'' said one analyst. For bulls, there are plenty of reasons to remain sanguine. The Hong Kong market is still cheap on an earnings basis when compared to the rest of the region, corporate profit growth is robust and property counters still look undervalued.