Air Carrier Captain reported while on a test flight observing slat malfunction indications resulting in a flap over speed and return to the departure airport.

Narrative

Conducting a test flight following a dual-engine change; we were climbing through FL330 for FL350 when both the PF and PM observed the red airspeed barber pole on their respective PFDs (Primary Flight Display) rapidly move down the display to cover our current airspeed by approximately 50 knots. Shortly after that; a LE (Leading Edge) EDGE SLAT ASSY EICAS caution was displayed and it was also observed that the flap position indicator had moved out of the UP position. We were approximately 300 KIAS at the time; so I slowed to below 250 KIAS. After slowing to below 250 KIAS; the EICAS caution was removed; the flap position indicator indicated UP and the red barber pole indication on the PFD retreated to normal condition. We suspended all flight testing tasks; and requested a PPOS (Present Position) hold and descent to FL150 for controllability checks. Extending flaps to 15 with an airspeed of 190 KIAS; all flying qualities were normal. We returned to ZZZ at 190 KIAS at flaps 15 and prepared for a flaps 25 landing. The anomaly never returned and the subsequent approach and landing were normal. [We advised ATC].