The visitors lost five for 28 in the second morning to open the door to Middlesex, who found an unlikely wicket-taker in Paul Stirling.
The Irishman's part-time spin removed Gareth Batty and last man Jade Dernbach to return figures of two for 48. Tim Murtagh's three for 54 was the best bowler's return.
Luke Wells fell four runs short of a back-to-back centuries as Sussex progressed to 218 for three against Warwickshire at Hove.
Wells hit a double century against Surrey last weekend and looked set for three figures again only to be bowled by Boyd Rankin for 96.
Rankin claimed the only other wicket of the session when Michael Yardy (32) edged behind.